Home / 1.4: Human Evolution and Emergence of Man

1.4: Human Evolution and Emergence of Man

Since the conditions under which life originated no longer exist & the initial products of biopoiesis not available to us in the from of fossils, the various steps through which life could have originated is largly a matter of conjecture.

Organic Evolution

Introduction – The term evolution was first used by Herbert spence to denote historical development of life. Whereas Acc to Darwinorganic evolution, ‘Descent with modification.’

  • It is irreversible, Unidirectional process which encompasses emergence, divergence & progress of life.

Meaning of Organic Evolution It is Gradual process of change in Gene frequency of population. It is the result of interplay b/w env & various factors including mutation, recombination, natural selection, isolation, migration, hybridisation & genetic drift. As env is ever-changing –  living beings modify & adapt accoding to the ever changing env.al needs, consequently it lead to change in the gene frequency of a population.

Types of Changes

  • Ontogenical – changes concerned with the history of development of idividua organism.
    • eg. sigle cell → multicellular adult
    • great importance to embryologists
  • Phylogenical – changes concerned with evolution of genetically related group of organisms.
    • evolutionary studies are concerned with these.

Premises of Organic Evolution

  • Env.al conditions in nature are even changing.
  • organisms have inherent character of change in order to adjust to the changin env. which is k/n as adaptability or adaptation.
  • Such adaptive changes in organism lead to the origin of new species
  • Since chagnes are due to adaptation, new speacies are always better adapted & more organised than their ancestors.
  • It is time consuming process
  • All the present day species had a common ancestor at some or other time of evolution.

Methods of Study of Organic Evolution

  • Palaeontology & Comparative Anatomy
    • only direct-evidence , in form of ‘Fragmentary fossilized bones’ whch make up the fossil record fo primate evolution,
    • Analysis of morphological similarities & differences has always been the basic aspect of evolutionary studies.
  • Molecular Method (Fundamental Similarty)
    • with the help of biomolecules , study of organic evolution is possible.
    • there is fundamental similarity in biomolecules in the course of evolution.
    • It suggest that how evolution occurs & also sometimes it can find out the sequence of evolution.
  • Embryological Evidence (Recapitutation Theory)
    • similarity of human embryo with fish embro in few intial weeks
  • Evidences from taxonomy
    • Grouping in same group based on similarites.
  • Geographical Distribution
    • Related fossils & living organisms are found in same geographical region.
  • Cytogenetic Evidence
    • Nucleosome G bands similarity
    • G band of ape chromosomes 12 & 13 perfectly match with G-bands of short & long arm of human chomosome 2 respectively indicating that human chromosome 2 is fusion product of ape chromosome 12 & 13.
  • Immunological Evidence
    • similarty in antigen antibody rxn
    • Similarity in human albumin protein of Human being &
      • Chimpanzee & Gorilla (95%)
      • Organ-utan (85%)
      • Gibbon (82%)

Mechanism of Organic Evolution

It involves change in gene frequecy of a population due to various factors such as migration, isolation, hybridisation & recombination, genetic drift, natural selection, mutation etc.

Theories of Organic Evolution – these can be divided into

  1. Pre-Darwinism – such as Lamarckism
  2. Darwinism
  3. Post – Darwinism

Theories of Organic Evolution

Pre-Darwinism

These theories are given by philosopher, politician, preachers & poets, scholars & scientist to answer when, where & how first life appeared. 

  • Mythological Theories – India, china, Egypt
    • Ancient India – followed the idea that universe & all living beings are made up of 5 elements (Panchbhutas) namely – Earth 🌍 (Purthvi), AP 💧 (Water), Teja 🔥 (Fire), Vayu💨 (Air), Akash 🌌 (sky)
    • Padma Purana – describes sequence of Evolution thr 10 incarcerations of lord Vishnu. 
    • In Ancient China -conceived that world was made up of primary element namely water, wood, fire & precious metal gold.
    • Egyptians gave the idea of Cosmic evolution involving creator, divine intelligence & water.
  • Theory of Spontaneous Generation – Ancient Greeks & Romans help that living org spontaneously generated from non-living substances. Ex
    • Thales → aquatic origin of life
    • Anaximander – origin of life from primordial fluid → after dried up life began
    • Aristotel – origin of life from inorganic stage to organic stage
      • Organic world consisting of plants, plant animal, animals
  • Theory of Eternity of Life – Carolus Linnaeus
    • Life has ever been in existence in the form as it exist today & will continue to be so forever
    • It has neither beginning nor end
    • God has created the species which are immutable & fixed.
  • Theory of Catastrophism (Anti Evolution) – G. Cuvier
    • After analysing fossilised remains he proved many fo these belongs to animals no longer in existence
    • He proposed “continuous process of catastrophes followed by migration from other regions.
  • In the 4th  B.C., Theophrastus, a Greek, has written a book on seed germination.
  • Some early Greeks believed that heredity among humans was a question of which sex dominated in the sex act.
  • Theory of Preformation – Every organism must therefore contain in its reproductive organs an infinite series representing all of its future descendants”
  • Ovists believed the female possessed this future of the series, and the spermists insisted it was the male
  • Linnaeus also suggested a two‐layered theory, which held that ‘‘the outer layer including the vascular system is derived from the father, the inner layer including the nervous system comes from the mother’’
  • That plants lacked sexuality was the conventional wisdom of the day.
  • End of the 17th c – the presence of sex organs in plants demonstrated by Rudolph Camerarius.
  • Hybridization experiments, finally culminated in Universal Law of Genetics by Mendel
  • Lamarckism – Theory of Inheritance of Acquired Characters

Darwinism

Post-Darwininsm

  • Discovery of Mendel’s laws of inheritance by Correns and Tschemark
  • Wagner – role of geographical isolation in speciation
  • Hugo de Vreis – Mutation theory
  • Research in population genetics proved that Darwin’s struggle for existence b/w individuals is not a very significant force in evolution
  • Unit of evolution is recognised as POPULATION and NOT INDIVIDUAL as Darwin believed
  • Neo Darwinism or Synthetic theory

Lamarckian Theory of Evolution

Introduction

  • Jean Baptiste de Lamarck, a French Evolutionist,
  • Book – Philosophie Zoologique (1809)
  • K/n as Theory of Inheritance of aquaried characters
  • he proposes that  changes in env. Brings about bodily changes in an individual energised by their inner urge.
  • Such changes are developed by use & are handed over to next generation.

Meaning of Organic Evolution – is Gradual process of change in Gene frequency of population. It is the result of interplay b/w env & various factors including mutation, recombination, natural selection, isolation, migration, hybridisation & genetic drift. As env is ever-changing –  living beings modify & adapt accoding to the ever changing env.al needs, consequently it lead to change in the gene frequency of a population.

Lamarck tried to explain the process of it in Following Manner :

  • Postulate I – Elan vital or inner urge (Theory of Growth)
    • Internal forces of life tend to ⬆️ the size of an organism by growth in organs & system.
    • eg. seed grows into a plant b/c elan vital, Zygote → giant creature
    • All factors of growth remain hidden in seed & zygote.
  • Postulate II – Theory of environmental pressure or spontaneous formation of organs
    • Chages in env. initiates need & want in the individual
    • Individual’s internal forces direct formation of new organs to meet the changes in env.
  • Postulate III – Theory of use & disuse
    • The development of organ is due to its use.
    • continuous use strengthens the organ & disuse has reverse effect.
    • Examples of Use : Giraffe, Horse, Blacksmith etc.
      • Lengthening of neck in Giraffe due to its continuous use to reach leafs & fruits of high rise trees.
      • Strong biceps in blacksmiths
      • Developement of strong leg bones & muscles in horse due to fast running on grass land after shifting from forest life to savannah life.
    • Examples of Disue
      • Elongated body & loss of limbs in snake due to continuous creeping through the hole
      • Occurrence of vestigial organ in man eg. External ear muscles, vermiform appendix in man due to its continuous disuse.
  • Postulate IV – Inheritance of acquired Characteristics
    • All the changes that organism acquire during the life time are transmitted to the next generation.

Analysis of Lamarckism

  • Evidences & Experminets against Lamarckism
    • wearing of iron shoes by females in chine doesn’t lead to reduced feet in next gen.
      • (3 feet shoe- Lotus feets – Being marriage material- started in 10th c ended in 20th c might be due to civil war in china)
    • Boring or ears & nose – no transmission
    • Mutilation experiment by Weisman
      • he amputated mice’s tail upto 22 gen. but no mice were born without tail in any gen.
      • hence he said that there are 2 types of cells – Somatic cells & Germinal cells
      • Mutation occurs in germinal cells, it can’t be transmitted.
  • Experiments in Favour of Lamarckism
    • Rearing white rats at 3℃-40℃ produces large body, long tail & hind leg & acquired characters were transmitted.
      • Further investigation suggested that the changes were due to mutation in germ cells.
    • Griffith’s Experiment – Putting a white rat on a rotator wheel causes dizziness. No further investigation was done. Dizziness was shown to be in various degree in offspring but it was not confirmed by others.
    • Power’s Experiment on Potatoes : potato→ harsh conditions→ altered str. → altered str. is trasmitted. but it was not supported by others.

Recent Controversy About Lamarckism

  • Experiment by Guyer & Smith
    • on antibodies against lens proteins of rabbit & showed that bodily changes can bring about formation of new RNA then → DNA
    • i.e Somatic Changes→ Antibody (Protein) → RNA→ DNa
    • RNA→ DNA is proved already
    • If protein → RNA proved then there will be no difficulty in proving it & in turn Lamarckism
  • Epigenomics involves changes in gene function that do involve changes in DNA sequence.
    • inheritance of food controlled coat colour in mice & env.ally induced heritable epigenetic changes involving alternative in HSP 90 (Heat Shock Protein)
    • however the evidence may become doubtful if it is found that methylation & HSP 90 like substances affect simultaneously both somatic & germ cells.

Criticism

  • Ist Postulate – some times there is reduction in size eg. Fern – Flowering Plant
  • 2nd Postulate – if the developement of new organs depends upon the desire then why man who have long desired to fly like births have not developed the wings
  • 3rd Postulate – heart & eye are not increasing its size in spite of its continuous use.
  • 4th Postulate
    • Augus Weisman experiments
    • Piercining of Ear & Nostrils in Indian Women
    • Wearing iron shoes in chinese female
  • he hadn’t differentiated b/w germplasm & somatoplasm.

After development of Darwin’s theory(1859), there formed two camps of scientist, one Pro-Lamarckian & other Pro-Darwinian

  • Pro-Lamarckian maintained the acquired characters are transmitted
  • Whereas Pro-Darwinians said that it is nature which selects fittest among existing variables & that aquired characters unless they are genetic in nature cann’t be transmitted.

Darwinism -Theory of Natural Selection

Introduction

  • Charles Darwin
  • in 1859 Darwin publised a book “Origin of Species by Natural Selection” or “The preservation of Favored Races in the struggle for life”
  • “The change in species by the survival of an organismal type exhibiting a natural variation, that gives it an adaptive advantage in an environment, thus leading to new environment equilibrium, is evolution by natural selection”.

Meaning of Organic Evolution – Acc to Darwin it involves gradual change in gene frequency which is inherited to next generation or descent with modification. ~~

Factors influencing Darwin

  • Theory of Uni& formitarianism by Charles Lyell – time period required for evolution
  • Ideas of Alfred Russell  wallace on Evolutionary Biology : tendency of variation to depart from original type.
  • Malthusian Essay on Population
    • Struggle for existence was derived from it.
  • Idustrial Revolution in England
    • I. which produced good & cheap goods→ flourished
    • so, in order to sum in & evolve organism must pose favorable variations
  • Empirical Evidences collected by Darwin during the Voyage
    • Faunae of Galapagos Islands – Turtle & Finches had shown great variation
    • The Pampas of Argentina – Fossils of extint animals
    • Darwin reasoned that species changes not only in time but also with geographical distance.
  • Morphological Evidence revealed by Darwin
    • hand of man, legs of horse, the paddle of the porpoise & the wings of the bat, were constructed on the same pattern & included same bones in relative position.

Theory – in his book “origin of species” (1859) he presented his theory under Postulates Based on 3 truth & 2 inferences;

  • Truths
    1. Reproductive capacity of animals & plants is very high.
    2. Population of species remains more or less same for every generation.
      • Inference that Darwin drew from these two thruths was that there would be a struggle for existence.
    3. Variations occur universally
  • Postulates
    • Overproduction (Excessive natural rate of muliplication )Living beings have an innate desire to reproduce more but population remains the same due to
      • Predation
      • Delicate & low viability
      • High susceptibility against biotic & abiotic factor.

deduction : Struggle for existence

  • Struggle for Existence
    • Intra-Specific Struggle
    • Inter-Specific Stuggle e.g. Plants & Animals
    • Stuggle against environment
  • Variations & Heredity i.e Inheritance of variation
    • Competition among organism compels them to change acc. to conditions so that they can utilize the natural resources & can survive successfully.
    • This creates variations among the organism. There fore, it is difficult to find out any two similar individuals.Deduction : Struggle for existence and variation & heredity together lead to survival of the fittest through natural selection.
  • Survival of the Fittest or Natural Selection – during the struggle for existence, only such individuals can survive who exhibit such variation that are proved to be more beneficial in facing the hardship or the rigors of env.
  • Origin of Species – The adaptaions are present & accumulated in the individuals of the species & ultimately lead to the emergence of new species from the old ones.

Criticism

  • doesn’t explain the effect of use & disuse or the presence of vestigial organs.
  • Darwin didn’t differentiate b/w the role of over specialized organs & the role specialized organs in different organism. In his words, over-specialized organs are useful to the organism. But this is far from the truth. Eg. Antlers of Irish Breed, Tusks of Jefferson Marmon
  • accounts for the survival of the fittest but not arrival of fittest
  • didn’t differentiate b/w somatic & germinal variations. & considered all variations heritable
  • Individual differences → 1° Importance & Mutation →2° imp. but mutation plays 1°role in evolution
  • Natural selection doesn’t expalin the evolution of terrestrial animals from aquatic ones
  • doesn’t explain the origin of variation
  • Physical scientist criticized that experimentation is needed to support the theory fo organic evolution
  • Indirectly accepted Lamarckism – inheritance of acquired characters

Difference Between Lamarckism & Darwinism

Introduction – first two theories of organic evolution given by – –

  • Concept of Lamarckism
  • Concept of Darwinism

Differences

ParametersDarwinLamarckism
Deals withvertical & horizontal aspect of evolutionConcentrate more on vertical aspect
Evidence based onEvolution of Common descentmissing
Diversity is d/t : Force of Evolutionstruggle for existence & favourable variationsinternal urge for growth I.e elan Vital & inner urge
VariationConsidered it as raw materialConsidered it as product of evolution.
Unit of EvolutionPopulationIndividual
Emphasison modification of populationon modification of individual
Inheritance Of traits that gave greater faintness & survivability & reproductive  potentialOf acquired traits
Role of NatureNature Selects Organism in a population for evolutionNature direct organism for evolution

Conclusion – Although Lamarckism & Darwinism are different theories yet they both consider evolution as a gradual process & were first to go against creationism. Both despite shortcoming contributed to the basic framework of evolutionary theory.

Synthetic Theory of Evolution

Introduction – Evolution is a gradual process of chane in gene frequency. After inability early theories to explain the cause of origin off variation. It was proposed

All the modern understandings in cytology, genetics, cytogenetics, population genetics, and evolution gave a way for the formulation of a coherent theory called “Modern Synthesis” around 1930s’.

It is a “synthesis” of “selectionist(1859-Darwin) & mutationist(1901-Hugo De Vries) views of evolution, Mendelian genetics, population genetics & molecular biology.

Historical Background For Development

  • After inability early theories to explain the cause of origin off variation
  • Therefore various evolutionist came up with studies explaining factors or cases of variation like mutation, isolation, genetic drift etc.
  • As this theory emerged with in order to eliminate shortcomings of Darwinism. Hence it is also K/n a New Darwinism or Post Darwinism.

Exponents of Synthetic Theory – in Dobzhanky’s Book Genetics & Origin of Species

  • Population Dynamic – Ernst Mayer, Julian Huxley, T.T. Thompson
  • Genetics & Mutation – Dobzansky, H.J. Muller, Hugo-de-vrise, G.L. Stebbing
  • Population Genetics – G.H. Hardy, W. Weinberg, Sewell Right.

Improvement over Darwinism

  • Rejected the theory of inheritance of acquired characters & gave emphasis on the gradualness of evolution.
  • Explain variation & mechanism of inheritance
  • Consider force of evolution other than natural selection such as gene flow, genetic drift, sexual selection etc
  • Explain evolution as change in allele frequency

Aim

  • To explain the mechanism of evolution
  • To explain evolution in term of genetical change in populations leading to the origin of new species.

Evolution Acc. The Modern Synthesis (Term by Julian Huxley)

  • Change in the allelic f. (numarical indication of genetic makeup) in a gene pool (sum total of all the allelic genes in population) of a given population from one generatin to the next.

Concept of Synthetic Theory

  • Variation
    • The difference in the closely related animals is called variation.
    • The change in the gene & gene frequencies is c/l genetic variations, which is raw material for evolution.
    • Small changes in the genetic material are fuel to natural selection. Genetic variation must be first produced & distributed before it can be acted upon by natural selection.
  • Unit of Evolution – Population
    • Individual can’t evolve as – alleles are variants of a gene & variation in population is b/c of the +nce of alleles & their inheritance
    • So, allele frequencies refer to only the whole group of individuals i.e populations. → There for only a group of individuals – a population can evolve over time.
  • Acc to Hardy & Weinberg frequencies of genes remains constant (I.e equilibrium) from generation to generation when
    • Population is large
    • Mating is random
    • Mutation don’t occurs
    • Population is closed -neither immigration not emigration
  • Evolution – occurs when equilibrium is altered. In any natural population, the genetic equilibrium is altered b/c Change in above factors.

Process of Evolution

  • Production & Redistribution of Variation – occurs in population by following factors
    • Genetic Mutations
    • Chromosome Aberrations 
    • Genetic Recombination
    • Migrations
    • Genetic Drift / Radom Drift / Sewell Wright effect
    • Gene flow
    • Hybridization
  • Action of Natural Selection on Variation
    • Adaptations – of phenotypes which are useful for an organism in a given environment
      • Organism having a genotype which gives it same advantages in particular env is said to be adopted.
    • Differential Reproduction – I.e one young produce more individual  than other
    • The org which have better adopted genotypes & phenotypes reproduced at a higher rate & leave more serving offspring’s in the next generation than other
    • Thus, Natural selection promotes the dev of more & more new adaptive genotypes & phenotypes. So is a creative force that spread novelty. 
  • Role Isolation & Speciation

Operation of Modern Synthesis

  1. Population develop genetic variations thr mutations, hybridisation, recombination etc
  2. Natural selection allows the favourable genetic variations to spread in the population thr differential reproduction in successive generations.
  3. The population are isolated geographically & reproductively & this leads to failure of inter breeding. When interbreeding doesn’t occur, the isolated population are grouped into new species.

Micro Evolution : Goldschmidt (1940) introduced the concept of micro-evolution to emphasize the fact that small mutations accumulate in population to bring about small changes in it & a/c for evolution of races & sub-species. Micro-evolution has not ceased example

  • The wisdom teeth in man – generally appears at the age of 16-18 Y ; not doesn’t appear even in the early or late 20s. It means that the man doesn’t need anymore
  • Menarche in Female – The avg. age of menarche is decreasing in all societies
  • Increase in height – Children of all ages of all societies are attaining greater height
  • Blunting of canines –The sharpening of canines in man is decreasing day by day.

Such changes are occurring b/c genes are having better expression in the modern env.

Macro-evolution : Large mutations accumulate in a population to bring about large changes in the population & account for evolution for species & higher levels of taxa 

Criticism of Synthetic Theory

  • This theory explains as it occurred in the past but fails to explain in terms of future direction
  • Change in chromosomal no. & genetic recombination is overemphasized & role of hybridization & genetic drift have been, undermined.
  • The role of migration in evolution is uncertain. Therefore its inclusion is superfluous.

Conclusion – The synthetic Theory is the most widely accepted theory & is constantly being refined by latest development in molecular genetics.

Role of Natural Selection

Introduction – It envisages differential rate of reproduction of different types of populations as result of different physiological, anatomical or behavioural changes by which biological traits become more or less common in a population as a function of inherited traits.

Proponent :

  • Darwin gave this concept of natural selection or survival of fittest :
  • He also provided two criteria for Natural Selection
    1. Differential reproduction or non-random mating
    2. Capacity of adaptation & pre-adaptation

Process of Organic Evolution

  • Genetic variation are created by mutations, migrations etc.
  • Some of these variations influence reproductive success
  • if, as result of genetic variation, some individuals contribute more offspring to the next generation than the others, it is the role of natural selection
  • Differential reproduction / Non random mating → elimination or selecting of certain traits → better adaptation to env. → change in gene frequency → organic evolution
  • Thus, natural selection is differential net reproductive success

Outcomes : Adaption

  • a result of natural selection is a change in allele frequency relative to specific env. factors
  • Such a functional shift in allele frequencies is c/a adaptation
  • During the struggle for existence only such individuals can survive who exhibits such variation that prove to be more beneficial in changing env.

Example : Survival & evolution of Human being

  • During course of evolution Australopithecus robustus got extinct & A. Gracile survived & got evolved into Homo sapiens.
  • Gene variation in +nt African population gives them resistance against the malaria.

Unit of Seletion – Individual

  • It is individuals who reproduce or don’t reproduce & who continually attemp to maximize their own reproductive success.
  • If total reproductive success of all members of a population continuously falls below replacement value the population will become extinct.
  • Evolution has no built-in mechanism to guard against extinction. Indeep, extinction is actually the rule -not the exception – in evolution,

Unit of Evolution – Population

  • the net result of all individuals reproductive success(natural selection) – in addition to the possible effect of mutaion, migration, & genetic drift – will affect the entire population.

Mode of Selection

depending upon the env.al change (rate & diversity) – hence selected accordingly. →sets of particultar pattern → mode of seletction

  • Direction Selection – When environment is slowly and gradually changing in one direction
  • Stabilising Selection – When environment is stable
  • Diversifying Selection – Extreme selection when drastic changes occur in environment

Conclusion – Though Natural selection played a role in evolution but there are other factors like genetic mutation, isolation, genetic drift which also lead to variation & evolution.

Genetic Recombination : Role in Organic Evolution

Introduction – It refers to rearrangement of genes through crossing over during meiosis in sexually reproducing organism resulting in new combination of parental genes in offspring. 

Process

Role in Evolution

  • The race or sub species becomes new species with further reproductive isolation and natural selection. Evolution happens when a new species which is better adapted to the environment is formed.

Advantage over other ways of Variation

  • Compared to mutation, genetic recombination has got greater role to produce variability.
  • Mutation & gene flow or migration can produce variability in a population with respect to single gene. But recombination can combine the novel alleles, which at first are likely to be carried out by difference in a single genotype.
  • It converts small initial stock of multiple gene variation into a much greater amount of genotype variation.

Evolution at Species Level

Species – group of interbreeding organisms, reproductively isolated from other species. Composed of subunit that are breeding communities, population.

  • Some degree of interbreeding (migration) is theoretically possible b/w all populations of that species.

The net result of all forces of evolution acting on all populations determines the fate of the species as a whole .

  • If sustained over a long period of time, gradual changes in allele frequencies b/w member populations can eventually lead to sufficient genetic differences,
    • so that fertile reproduction is no longer possible.
  • We then may recognize a new form of life having arisen from one species “splitting” and producing new species, a process called speciation.

Isolation & Speciation

Introduction – The significance of reproductive isolating mechanism is first time Dobzhansky in 1937.

Types of Isolating Mechanism

  • Acc. to Mayr. the way new species are first prduced involves some form of isolation.
  • no of isolation mechanisms may also prevent organisms from interbreeding amongst each other
    • Geographic isolation
    • Behavioral isolation
    • Mechanical isolation

Classification of isolating mechanism

  • Pre-mating or prezygotic
  • Post-mating or postzygotic

Function – 

  • ↑  the efficiency of mating.
  • Where other related species coexist. non specficity of singals may lead to wasteful coutship & delays, even where no heterospecific hybridisation occurs.
  • it protect the intergrity of the genetic system of species.

Basic Concepts & Term in Evolutionary Biology

Convergence & Parallelism

Introduction – sometimes two organism may show close relationship with respect some morphological features & pattern. Such similarities may be due to parallelism or convergent Evolution.  

Convergence

Definition – it is used to refer Similar evolutionary development in unrelated organisms or those that differ in direct ancestry due to similar evolutionary pressures.

Explaination

  • convergent evolution refers to evolution of superficially similar traits by unrelated species as a result of adaption to similar ways of life.
  • It is a process in which two unrelated group of organism, living in similar but separate environmental conditions, living in similar but separate environmental conditions, develop a similar appearance & life style.
  • That means, similar environmental demands make for similar phenotypic responses

Features

  • The development of similar forms in divergent evolutionary lines is termed as convergent evolution.
  • It ordinarily applies to one or a few charcterstics rather than to the overall makeup.
  • Homoplasy : means similarity in appearance but not in origin ;
    • result of convergent evolution

Example

  • Development of wings in Birds & bats
  • Streamline shapes of whale & Fishes converging into the same env.
  • Similarities in retina in eyes of some quite different organismsOnly rod (more sensitive to dim light) are +nt in some deep sea fish, bats some lizards & snakes, guinea pigs, whales & some lemure
  • The tarsier & humans both evolved large orbits for large eyes, along with small noses, as less dependence on the sense of smell was the trend.

Parallelism

Definition – Parallelism is usually applied to two species of organisms that were similar in origin, and that remained similar as they evolved in-dependably  into two different species for adaptation with both environmental & biological conditions of their respective habitats.

  • it is similar adaptations in organisms which share common ancestry.

Example

  • Old World Monkeys & New World Monkeys –separated 35 million years ago from a prosimian ancestor
  • Absence of tail in man, great apes & gibbon. They has same ancestor & tail must have lost in course of evolution in separate line but in parrllel fashion
  • placentals and marsupials, which have followed independent evolutionary pathways following the break-up of land-masses such as Gondwanaland roughly 100 million years ago.

Parallel Organism

  • resembles one another me or than their common ancestor
  • Their similarities are due to adaptation to similar conditions

Therefore, in Parallelism two related species independently take comparable ways of life. 

Reason of Convergence & parallelism

The reason for both the convergence & parallelism is same. The organism in order to survive in similar env., must develop similar biological str.

Parallelism, like convergence, is a matter of adaptation under the control of natural selection .

Conclusion – therefore, it is necessary that factors of parallelism & convergence must be taken into account of which are likely to result in structural resemblance of evolutionary development of related & unrelated group.

Homology & Analogy

Homology

  • Homology means similarity in origin. (Implying the common ancestor)
  • Organs which are similar in origin & not function are homologous
  • Parallel evolution following divergences of lines tend to result in homologies.
  • Ex. Forearms of vertebrates – Forelimbs of vertebrates
  • Homoplasy : means similarity in appearance but not in origin ;
    • result of convergent evolution
  • Serial Homology : Similarity in two structures within the same organism Ex. Hands and Legs in man

Analogy

  • Analogy means similarity in function but not in origin.
  • Organs which are simiar in function & not origin are analogous.
  • Analogies & homoplasy are result of convergent evolution
  • ex. Wings of birds & wings of insects

Adaptive Radiation

Introduction : The evolutionary spread & differentiation of the descendants of one type of animal, of whatever level of classification, is known as Adaptive radiation.

Definition

  • Acc to H.F Osborn, coiner of term, Adaptive relation is with great variety adoptions to different niches.
  • Bettner Janusch : Rapid increase in no. & types of any evolving groups of organisms into several distinct eco niches to which each particular group is adapted particularly is called Adaptive Radiation. 
  • Acc to John Relethoford – it is “The formation of many new species following the availability of new env or the development of new adoption.

Meaning & Features

  • The evolution which result in production of new adaptive types through a process of population fragmentation & genetic divergence is k/n as macroevolution.
  • This results in spliting of a population of a species in two different groups & several subgroups each exhibiting changes in definite adaptive direction. This phenomenon is k/n as adaptive radiation
  • Opposite of convergence and parallelism where different organisms arrive at common adaptation, radiation refers to the way a species evolves into progressively dissimilar organisms

Example

  • Adaptive radiation of reptiles in to the mammal, avian, pisces, borrowers & arboreal.
    • During paleocene epoch, cold blooded reptiles like Dinosaurs could not adopt & therefore got extinct
    • mammals evolved into different types to adjust to new env.
    • Primates took to trees (arboreal), bat to air (avian), whales to water bodies, carnivores became adapted to hunting, rodent for borrowing etc.
  • Mammals – Arial, arboreal, cursorial, fossorial, aquatic etc.
  • Primates exhibit radiation in body size, group size, brain size etc.

Methodology to Study Adaptive Radiation

  1. Morphological features of fossils
  2. Comparative anatomy of living forms

Mechanism of Adaptive Radiation

  • It operates above the species level & results in formation of new genera or family or order.
  • The changes in organisation occurs due to sudden mutation of large size which are named macro mutations
  • Micro mutations occur in a group of individuals which have netered a new adaptive zone free form competition.

Features of Adaptive Raditions

  • results in evolutionary changes
  • produces group of specially adapted animals among divergent stock
  • leads to specialization in particular direction which result in over specialization & ultimately extinctiction .
    • b/c over specialized forms are unable to modify when they enter a new adaptive zones.
  • It involves transition from generalised to specialised characteristics
    • Ex generalised mammalian limb has 5 fairly flexible digits adopted for many possible functions (grasping, weight support & digging)

Adaptive Radiation & Evolution

  • It illustrates that when a new form Aries, it will diverge into as many variations as follows, depending on
    1. its adaptive potential
    2. The adaptive opportunities of the available zones
  • The structural & functional ability of the body of an organism revels that adaptive potential of an organism
  • The ecological niches of the available zones may have their own potential to provide adoptive opportunities in the form of resources such as food, shelter & space.

Features

  • It involves transition from generalised to specialised characteristics
    • Ex generalised mammalian limb has 5 fairly flexible digits adopted for many possible functions (grasping, weight support & digging)
  • It reveals that evolution is not the process of superior animals triumphing over the inferior ones but it is rather, a series of alternative strategies that have arisen in response to inconsistent env.

Conclusion – thus it is One of the basic patterns of evolution. It reduces competition, helps in bettter exploitation of resources & produces new diversity among organisms.

Mosaic Evolution

Introduction : The concept that major evolutionary changes tend to take place in stages & not all at once is mosaic evolution.

Definition – A proces of differential evolution of component parts ,I.e rate of evolution of one functional system various from other,  of an organism or a stucture is c/a mosaic evolution

  • Acc. to this principle, the evolution of the species tends to be inconstant & asymmetrical.

Examples

  • Bipedalization followed by Encephalization : the early evoltution of bipedalism in Australopithecus & modification of pelvic girdle took place well before there was any significant change skull or brain size.
  • Mammalian evolution, especially during Mesozoic, provides a clear & well understood ex. of mosaic evolution

Importance

  • plays key role in macro evolution.
  • It brings stage by stage changes in different parts of organs
  • It sets a flexible framework within which different ogans have to grow & develop in time with the development of other part of the body
  • It shows not only differentical evolution but also overall adaptive pattern of evolution of an organism.

Dollo’s Law

Introduction – In 1893, Belgian Palaeontologist, Louis Dollo gave the Doctrine of irreversibility & irrevocability of evolution. He proposed that the structure that changes its form in the course of evolution is irreversible  to the extent that true atavisms or throwbacks to earlier forms, don’t occur in detail

Explanation – Doll’s rule seeks to explain the course of evolution. This rule predicts that evolutionary changes can’t be reverted back b/c every specie is the product of env & similar kind of env can’t be replicated.

Examples

  • Cetaceans (whales & dolphins) may have been returned to aquatic env. in which vertebrate first evolved, but they haven’t once again become fish.
  • Races, species, genera are formed over a period of time through changes in genetic composition. All these alterations cannot be reversed back to their original stock.
  • Throughout Pleistocene, human adapted to changing environmental conditions with changes like lower jaw, dentition, erect posture, brain size etc. which cannot be reversed.
  • Fish lost its gills which will never come back. taipolol
  • If a premolar is lost in course of evolution then it will never recur again as a premolar in subsequent evolution.

Thus, Number of adaptive characteristics like increasing overall size of organism or changes in number of vertebra in the backbone of reptiles require the integration of a series or a sequence of process in the evolutionary change.

Exceptions – But this rule does not mean that re-evolution of a certain character similar to an ancestral form never occurs.

  • E.g. certain extinct flying reptiles had wings. But in birds and bats, wings evolved independently.
  • After flying reptiles becames extinct wings & the adaption to fly recurred in two other distinct lineages namely lineage of birds & lineage of mammals (ex bat) 

Also the reverse mutations, (Single step backward) returning to ancestral condition sometimes occur

Reasons

  • the many different evolutionary details prevents a complete reversal of evolution.
  • change in of the natural env. in exact step-wise reverse order is exceedingly rare;
  • if a previous allele has been entirely eliminated from the gene pool, a reversal of evolution would also require precisely the correct reverse mutattion just as the env.al condition also reverse in proper order.:
  • When the structural changes are responsible for functional specialization, it confines a species to a narrow and restricted environment;  there is no possibility of reversion to primitive organization from which it originally derived.
  • Because such evolving groups have tendency to be more specialized and acquire more progressive useful variations Cal which leads to irreversibility of evolution.

Criticism

  • No logical or substantive similarity to laws of physics or chemistry
  • Irreversibility is a descriptive generalisation but not a law of nature.

Cope’s Rule

Introduction – Edward de cope, an American paleontologist, on the basis of extensive study of fossils of mammals gave this rule.

Cope’s Rule – The rule states that organisms have a tendency towards increase in size during evolution.

Examples

  • Evolution of horse, camel & all other herbivores exhibit ↑  in size so they attained gigantis,
  • Other mammals like tortoise & crocodiles exhibit the same phenomenon
  • Dinosaurs provide the most interesting example of ↑ in size.

Exceptions

  • Evolution of no. of mammalian lineages don’t show tendency towards gigantism
  • Insectivorous mammals
    • Mole can’t be large b/c that will be a difficulty in borrowing
  • Flying bat – its flying habit impose the limitation on the body size.
  • Carnivores attained gigantic size in Pliocene & Pleistocene but show ↓  in size thereafter.

Reasons for ↑  in size

  • It is now presumed that ↑  in size in different mammalian lineages may be account of different seasons. In Pleistocene, it could be a device to ↓  body surface to conserve body heat in cold env.
  • Among carnivores ↑  in size facilitates capturing of large sized herbivores on which they feed.
  • For egg laying mammals this would be a device to ↑  their fecundity that is no. of offsprings produced will be more due to more eggs in a large body.

Criticism

  • It’s Not a law of nature
  • Only descriptive generalisation
  • Does’t apply to living organism on earth 

Guse’s Rule – Principle of Competitive Exclusion

Introduction – Guse’s Rule or competitive exclusion was originally Explained by Russian Biologist Gause.

Rule – It state that if two organisms occupy the same trophic level in the ecosystem, they try to reach equilibrium through different strategies reducing the other to point of extinction

Reasons / Explanation

  • If two species occur at a same trophic level, then they are likely to compete for food, Isto space etc. Adaptive radiation by one or more species may occur over a period of o time. Consequently they come to occupy separate niche within the trophic level thus minimizing the extent of competition.
  • Alternatively if the competitors occupy the same trophic levels (food levels) in a strongly overlapping niche and equilibrium situation may have been reached where neither succeeds, then one of the competitors decline in number to the point of extinction

Experiments

  • It is difficult to study this phenomenon in wild. He worked in laboratory population & the result was as follows.
  • Paramecium aurelia & P. caudatum exist at same trophic level & complete with each other.
    • When cultured separately both prosper.
    • When cultured together, caudatum is eliminated & aurelia survives.

Example

  • Finches of Galapagos Island – through adoptive radiation ancestral finches give rise to many relatively more specialised descendant finches which show morphological differences in shape & size of their beak structure.  

Conclusion: This principle is universally accepted. It tries to explain organic evolution through the principles of struggle for existence & survival of the fittest.

Biological & Cultural Factors in Human Evolution

Human being has been shaped through the synergetic interaction of genes & Cultures. Both influence each other as they are interdependent. Thus human evolution is an intriguing interplay of biological evolution & cultural evolution. Chalet Lunsden & E.O Wilson dubbed this interaction as gene, culture Co-Evolution.

Biological Factors in Evolution

  1. Erect Posture & Bipedal Locomotion
  2. Remodelling of face & teeth
  3. Expansion of Development of Brain

Cultural Factors of Evolution

  1. Language & speech
  2. Tool Making & Use

Biology always influeces the culture. Biological development of man preceded his cultural development. 

  • Environment Forces & Genetic factors were the major elements influencing the biological evolution of man.
  • The biological progress of Man in due course laid the foundation for origin of culture.

Example

  • Erect Postures & Bipedalism → tools for hunting & food gathering
  • Progressive change in size & complicity of Human brain → capacity for symbolic through & communication (Language) → Social behaviour

After this period rate of cultural evolution began to overtake biological evolution & also influence biological evolution.

Cultural Influence On Biology

  • the knowledge & technique developed by man thr cultural evolution influence & modify the biological evolution faced by him. The result of cultural evolution have brought changes in the “Gene Pool of human species

Example

Influence of Biology on Culture

  • To solve the problem of delivery of child with enlarged brain ; resulted in enlargement of public opening, widening of hip region & prolonged infant dependency which ultimately resulted in

Example

Progressive changes in brain

  • created favourable condition for symbolic though & communication leading to
  • Development of language
    • Three areas crucial for human linguistic ability located in cortex
      • Broca’a Area
        • this area activates muscles of jaw, lips, tonge & larynx
      • Wernicke’s Area
        • brain site for verbal comprehension
      • Angular Gyrus
        • situated next to wernicke’s area
        • serving as link up b/w parts of the brain that receive stimuli from the sense ogans of touch, hearing & sight.
    • It leads to Regular Transmission of knowledge
      • individual to individual 
      • Group to group
      • Generation to generation.
  • Invention of Art of Writing – Speed of Transmission Enhanced
    • Helped man in concentration of knowledge & spread thr time & space I.e Collective Learning

Thus, through cultural achievement man has became 1st species in the world to guide his own evolution.

Transfer of Information – Biological based behaviour (Biological information) requires enormous no. Of generations to spread. Whereas cultural innovations especially in the era of info revolution spread very quickly.

Conclusion – In the Bronze Age & Iron Age that followed with the dev of metallurgy, Uran revolution had set in with exceptionally fast & progressive cultural take-off leaving behind the biological evolution. Thus two aspect of the single process of human evolution interacted with one another hence evolution of human being is considered as Bio-cultural evolution

Biological Factors which Made Human Capable of Culture

The process of human evolution from the time (~ 5mya) of split of humans from apes has involved progressive changes in human biology for instance – erect posture, bipedalism, encephalisation. This is turn made culture a possibility & reality for humans.

Biological changes that Made Culture Possible

  • Changes in position of foremen magnums to more forward position
    • Upright posture, freeing hand for work
  • Erect Postures & Spinal Arch
    • Freed Up hands
      • Other actives for tool making, crafts art etc
    • Narrowed Birth Canal – less developed children birth → intensive care – ↑ dependence → dependence on ♂ → m’age , kinship
  • Pelvic Gridle
    • change
      • Broad basin shaped pelvic, shorter & broad ilium, shorter girdle
      • Spinal – ilium point closer to femur – ilium point for weight transmission.
    • Sexual dimorphism in ♂ & ♀
    • Let to sexual selection / preferences  , assertive mating
    • Longer gestation period – socialisation
  • Change in Diet, smaller Intestine & Smaller Dentition (Teeth)
    • In conjunction with use of fire
    • Cooking – less time spent on food acquisition + digestion → art & culture
    • More energy from food → more energy for brain to utilise → development
  • encephalitisation – ↑ in cranial sie
    • Language centre – Language + co-operation
      • E.g Homo sapiens as homo lequins
    • Complex though, Planning, Strategising, Cultural Progresion – religion
    • Frontal lobes → organised behaviour into sequels
    • Motor association area → control fingered & thumbs → tools
    • Planning → making tools in advance
  • Hand & foot anatomy : Limbs
    • Changes –
      • Tibia, fibula , femur elongate with developed muscle
        • Bipedal & upright posture
        • Forster running, hunting easier & more efficient
      • Patella, long thing & femur + Olorcoplanar
      • Arhces – Medislateral arch,
    • enabled erect posture,
    • Running – capable of chasing down prey
      • H&G life style
    • Powerful curse → Cary heavy objects
    • Ability to fully oppose the thumb to all the fighter is uniquely human → small objects held b/w finger & thumb
  • Hand Anatomy
    • Precision grip, opposable thumb
      • Tool making , art sculpting
      • Throwing of spear → hunting made easier
  • Facial changes + Musculature
    • Expression, socialisation & communication

Conclusion – however, dual theory of (inheritance) evolution – bio cultural evolution.

Unicentric V/s Multicentric : Source of Human Evolution

Introduction – Evolution is the process of gradual change from simple to complex form, from homogeneity to heterogeneity. Different scholar have came up with different theories to explain the centre of origin of Homo Sapiens. So this Unicentric v/c multi-centric is debate about one or multiple centres of origin of Homo sapiens

Views & proponents of respective theories are following

Unicentric Theory

Acc to this view – Homo sapiens originated in Africa only & then spread to other parts of the world.

  • Darwin – ‘Descent of man‘ – out of Africa theory – unicentric theory of origin.

Supporters

  • Kenn et al (1987) in Nature, after studing mitochondril DNA concluded that ‘ mother of sapiens’ lived in Africa b/w 0.29 mya to 0.14 mya
  • Neil & Livshitz (1989) : on the basis of 148 gene markers came out in support of it

Multicentric Theory

Acc to this view , Homo sapiens originated simultaneously from different centers of the world & not necessarily only Africa

Supporters

  • Paul Graves raised doubts about conclusion of Kenn et al
  • Kramer (1991) in an article published the American Journal of Anthrology, on the basis of study fo a Mndible discovered from Sangarian, Java come out with multicetric therory
  • Simmons & Smith (1991) studies all the discovered fossils of Homo sapiens & published an article in Current Anthropology in support of Multicentric theory of sapiens.

Conculusion – Debate is still going on But one thing is certain, since oldest fossils of erectus have been found in Asia (Java) are also the youngest, if sapiens have evolved from erectus, which is undebatable then Asia must be another center of human evolution. But the conclusion is tentative till more fossils are discovered from the region.

  1. Replacement model suggest that all the homo who are alive today have common ancestor that migrated from Africa( out of the Africa 2 theory). We replaced all other homo surviving those days and now only single species of homo, i.e- us exist today….. so we had same mother
  2. Multi-origin theory suggest that we evolved from our earlier ancestors (like erectus surviving whole of old world) from different places, not only from Africa(not only from homo ergaster) so we all had not common mother but different at different places.

Earlier it was a hot debate but advancements in applied anthropology and dating techniques has solved the debate to a greater extent.

The main concept used is— MRCA(most recent common ancestors) which is determined through analysis of 4 type of DNA- Using the technique like PCR, blotting, etc

  1. Mitochondrial DNA
  2. Nuclear DNA
  3. Ancient DNA
  4. Y- chromosome of fossils

This has suggested the higher conformity towards the replacement model which suggest that , Yes we (homo sapiens) had common mother and we spreaded out from AFRICA about 1.5  lakh to 90000 years ago and further spread old and new world in late pleistocene era. So before the deepest node of our MRCA( about 90000 yrs ago) ….. probably we can say that yes…. We had common mother (means yes to out of AFRICA 2 theory)

Note- out of Africa 1 theory is related to spread of homo Erectus.