8 Methods of studying human evolution

Mr. Vijit Deepani and Prof. A.K. Kapoor

Contents:

• Human Evolution: A brief outline
• Methods of studying human evolution
• Comparative morphology and anatomy
• Comparative embryology
• Biochemistry and comparative physiology
• Genetic analysis
• Palaeontology
• Biogeography
• Taxonomy
• Summary

Learning Outcomes:

The learner will be able to understand:

• the direct methods of studying human evolution.
• the indirect methods of studying human evolution.

Human Evolution: A brief outline

Evolution (derived from Latin word- evolvere – means ‘to unfold’) has led to the development of vast panorama of diversity of organisms on earth. As a process, it simply aims to reveal hidden potentialities that lead to an orderly transformation from one condition to another. Evolution is also termed as ‘descent with modifications’. Evolutionary processes bring out changes in certain characteristics that eventually lead to development of dissimilarities between ancestral and the descendant population. Evolution caters to populations and not individual organisms.

Human evolution, from genetic point of view, can be viewed as changes in gene frequency (of human gene pool) from one generation to the next with time. As a result of the process of evolution, humans had to pass several developmental stages so as to occupy the present taxonomic position (that distinguishes man from other organisms).

Campbell (1998) suggested that the dynamic stability of all the genetic components (termed as gene pool) of a population makes possible adaptation to the environment as well as modifications of such adaptations in the presence of environmental change in the future and the absence of such modifications can lead to extinction of population. He further suggested that an appropriate balance must be established between stability and flexibility and it is an alteration in form of this balance that, among other characteristics, characterizes human evolution.

Darwin believed that man has evolved and descended from non-human ancestors and certain primitive forms were common ancestors of man and apes. He even suggested that the common ancestor was a hairy, tailed quadruped, probably arboreal in habit (Das, 2008). There must be certain convincing evidences for the episode of descent with modification and the following section throws light upon them.

Methods of studying human evolution

There are several evidences or proof (most of them are circumstantial) which support that organic evolution has occurred. The different phenomena; which provide proof for evolution (directly or indirectly) support one another. All these evidences, which point towards the same conclusion, are drawn from different sciences (such as embryology, palaeontology, morphology, physiology, embryology, geography, taxonomy, genetics and so on) and these evidences, in turn, act as crucial methods to study human evolution. Thus, the following methods are used to study human evolution:

• Comparative morphology and anatomy
• Comparative embryology
• Biochemistry and comparative physiology
• Genetic analysis
• Palaeontology
• Biogeography
• Taxonomy

Comparative morphology and anatomy

This method deals with the study of the morphological similarities with emphasis on structural relationship between different organisms including man. Several anatomical peculiarities such as the connecting links, homologous and analogous organs, vestigial organs and atavism present in various animal groups offer crucial means to study the course of evolution as these evidences can be explained on the basis of evolution.

Morphological examination of several organ systems among vertebrates exhibits that these are constructed on the same fundamental internal structure (characterized with similar ontogenetic histories) although they exhibit minor differences because of their adaptive modifications to the diverse mode of living. This similarity is termed as ‘homology’ and the organ are called ‘homologous’. The pentadactyl constitution of the fore limbs among several animals (vertebrates) offer typical example of homologous organs. Figure I provide a glimpse of homologous structures in certain animals and in these animals, the same internal structure developed along different directions owing to adaptations to different needs. This is termed as divergent evolution.

Fig I: Homology among the bones of the forelimb.

The same basic internal structure is found in the forelimbs of humans, cats, bats, porpoises and horses, although these structures exhibit differences in form and function. (Source: Raven et al, 2005)

Homology exhibit common ancestry i.e. these vertebrates must have had a common ancestor with particular forelimb prototype. Homology is exhibited in every organ system from fish to man (Jaiswal, 2013). In contrast to homology, analogy relates to convergent evolution. The analogous organs are anatomically dissimilar but because of adaptation along similar lines they perform similar function.

Vestigial organs also offer crucial evidence to study course of human evolution. Vestigial organs are imperfectly developed, rudimentary (or degenerate) and non-functional organs which might have been functional in related ancestral forms. Humans have many vestigial organs such as vermiform appendix, wisdom teeth, plica semilunaris, external ear muscles, coccyx (vestigial tail vertebrae) and many others. The presence of non-functional appendix in man exhibits that ancestors of man (the early primates) had a much coarser diet (resistant vegetable matter) but with gradual change in dietary pattern, the caecum and appendix have become reduced and non-functional.

Fig II: Vestigial organs in man

(Source Link: https://shoker23.files.wordpress.com/2015/03/382_vestigial-rgans.png)

The phenomenon of atavism also provides crucial evidence for human evolution. Jaiswal (2013) defined atavism (or reversion) as reappearance of an ancestral trait in an organism or in the group of organisms, which do not occur normally or which represent the reminiscent of normal structures possessed by the individuals of other groups. This phenomenon is well exhibited in man as illustrated by presence of cervical fistula, dense body hair, additional mammary gland (besides a pair of pectoral mammae) and enlarged canines.

Connecting links also provide crucial insight into path of evolution. They represent intermediate forms between two animal groups and exhibit traits of both the groups. For e.g. egg laying mammals are primitive mammals which represent connecting link between reptiles and mammals as the former possesses characteristics of both reptiles and mammals.

Comparative Embryology

According to Reddy (1992), embryology is the study of individual development in its earlier phases. The comparisons of development periods of different organisms can offer crucial anatomical evidence supporting evolution. Ernst Haeckel proposed the ‘Recapitulation theory’ or ‘Biogenetic Law’ as he observed a generalised developmental pattern between the embryos of different animal groups. The Biogenetic Law stated that Ontogeny recapitulates phylogeny i.e. an organism during its developmental period resembles embryogenic stages of its ancestor. This proposition is also called phylembryogenesis.

Fig III: Embryos of different animals exhibiting resemblance

(SourceLink:https://image.jimcdn.com/app/cms/image/transf/none/path/sf2bda65f6bb61a9d/image/i1d0e64f26073a9aa/version/1410116500/image.jpg)

Human embryo in its development stages obeys recapitulation theory. A human embryo, early in their development possesses – gill slits and well developed notochord- fundamental chordate traits. With gradual development, it exhibits similarities to a fish embryo in having paired aortic arches, two chambered heart, pronephric kidney and a tail. Later it resembles a reptile embryo and eventually possesses mammalian traits. At a later stage, every human embryo posses a long bony tail (carried to adulthood as the vestigial coccyx at the end of our spine) a fine fur (called lanugo). These relict developmental stages strongly support biogenetic law and emphasize that our development has evolved.

The divergence in development stages manifests sooner in widely different groups of organisms and later in case of closely linked organism groups. The embryos of different vertebrates exhibiting differences in advanced stages present remarkable similarities in the earlier stages (Reddy, 1992). The similar anatomical structure in the adult forms, the greater is the similarity in their developmental stages.

Biochemistry and comparative physiology

The similarities in the biochemical composition and physiological activities of organisms offer the most convincing evidence of descent from common ancestors (Jaiswal, 2013).

Biochemicals: Living beings are characterized with similar types of biochemicals, biochemical reactions and body functions. The chemical composition of all living beings is based upon four principle chemical elements – carbon (C), hydrogen (H), oxygen (O) and nitrogen (N). Even the hormones of vertebrates are structurally and functionally similar. In chimpanzees and humans cytochrome-C molecules are identical.

DNA and genetic code: Except for differences in the sequence of nitrogenous bases, the chemical composition of DNA is basically same in all living beings (Jaiswal, 2013). The same genetic code (triplet code) is prevalent from viruses to man.

Fig IV: Relative similarities between Human DNA and other vertebrate’s DNA

(Source: Jaiswal, 2013)

Blood serum tests and comparative serology: Blood groups also help to trace relationship between living organisms. Humans exhibit ABO blood group system, apes possess blood groups A and B but monkeys lack them. This suggests a closer relationship among the former groups. Further, beta chain of haemoglobin of human and gorilla, human and pig and human and horse differ in one, ten and twenty-six amino acids respectively. Dr. George H. F. Nuttall utilized the precipitation method to assess inter-relationship among different animals. Serum or blood protein tests emphasize that our nearest relatives are apes, followed by old world monkeys, new world monkeys and tarsiers.

With the help of genetic analysis, the basic mechanism of inheritance, transmission of physical characteristics from one generation to next and the impact of evolutionary changes in the past can be understood. Genetic analysis of living organisms thus offers conclusive evidence to study human evolution. It has been established now that genes are quite constant and are inherited unchanged from generation after generation (Jaiswal, 2013). But mutations cause changes in the genetic makeup of organisms which further leads to variation among similar living organisms.

Mutations cater to permanent heritable modification or changes in the genetic material of individuals. They are produced by changes in chromosomes (polyploidy and aneuploidy), chromosome aberrations (deletion, duplication, inversion and translocation) and genes (number and sequence of nucleotides). Mutations develop and maintain variations within populations and even add new genes and alleles into the gene pool. Thus, the raw material for evolutionary change is mutation.

The genetic differences among humans is about 0.1% and the examination of the same aspects of chimp genome exhibit a difference of about 1.2%. The genetic difference with gorillas is about 1.6% and the genetic difference of 3.1% distinguishes humans and the African apes from the orangutan. The strong genetic similarities between humans and the African great apes enabled the scientists to predict that Africa was the likely region where the human lineage split off from other animals and where the common ancestor of chimpanzees, humans, and gorillas once inhabited.

Accumulation of mutations, over time, leads to production of new varieties and races. It is also possible to synthesize new life forms by hybridization and these newly developed life forms support evolutionary episode through the process of mutation (Reddy, 1992).

Palaeontological evidence

Palaeontology refers to the study of fossils or remains, traces and impressions of past organisms found in the rocks of different ages. Reddy (1992) emphasized that the paleontological record relies upon the time tested principles of geology which exhibits a succession of transitional stages in the development of animal groups through time and presents a temporal sequence of life showing how the early forms look, what changes occurred in their morphology and how they differentiated in time and in space and so on. The available paleontological record exhibits objective evidence for discerning evolutionary trends as those located in the upper strata have comparatively more complex organization than those located in the lower strata.

Fossils are prominent paleontological records and the study of fossils reveals the existence of life in past and illustrates the course of evolution of plants and animals. The study of fossil records provide direct evidence in support of human evolution. Fossils obtained from different geological strata relate to different genera. Transitional fossil forms are observed between different existing groups. These fossil forms are called ‘missing link’ as they serve to bridge between the two large distinct groups and throw light upon the courses of evolution. Archaeopteryx fossil exhibit that birds have evolved from reptiles. Eugene Dubois considered Pithecanthropus erectus (ape man with erect posture) as the missing link between apes and man, as the former stood in the direct line of evolution of modern man.

Fig V: Dubois Java man (Pithecanthropus erectus) indicated that hominid bipedal locomotion evolved prior to the large brain of modern humans. (Source: Marshall, 1999)

On the basis of examination of living and fossil primates, it has been concluded that modern day primates (including monkey and man) have evolved from a common ancestor and split off from the main stock in Oligocene period. African ape man (Australopithecus africanus), Java ape man (Pithecanthropus erectus), Peking man (Sinanthropus pekinensis), Heidelberg man (Homo heidelbergensis), Neanderthal man (Homo neanderthalenensis), Cro-Magnon man (Homo sapiens fossilis) and Modern man (Homo sapiens) constitute major fossil record, which play a crucial role to assess the course of human evolution.

In order to assess evolutionary relationship between fossil remains, it is essential to assess the age of fossils. The knowledge of relative and absolute age of fossil forms is vital if we are to develop a phylogenetic sequence as the prime motive is to reconstruct an evolving lineage of individuals and populations. Two prominent dating methods – Relative and Absolute (or chronometric) – are utilized to determine the age of fossils. Relative dating methods are centered upon the knowledge of stratigraphy while the chronometric dating methods provide the age of fossils in ‘absolute terms’. The latter developed as a result of the discovery that certain naturally occurring radioactive elements decay at constant, known and measurable rates into other known elements (Campbell, 1998).

Complete evolutionary history of certain animals (such as man, camel, elephant and horse) have been traced out by examination of fossil records. Geological time scale depicts the distribution of animals and plants in geological time as a sequence of events through different ages. The geological time scale has been subdivided into six major eras, which are further ramified into several periods. Tertiary period of coenozoic era is termed as the age of mammals, birds and angiosperms while Quaternary period of the same era is called the age of man and herbs.

The palaeontological evidences have certain limitations such as too many gaps in the fossil record in view of the existence of unfavourable climatic conditions in many regions for the preservation of organic remains through fossilisation, most fossil remains were found by accident and fragmentary in nature and mostly represented by bones and teeth, problems of dating and interpretation, absence of complete sequence, etc. but they offer conclusive evidence to study the course of human evolution (Reddy, 1992).

Biogeography: Evidence from geographic distribution

Rastogi (2004) defined biogeography as the study of distribution of animals and plants in space or earth surface. These geographic evidences constitute another crucial method to study and assess evolution. The different regions on earth exhibit distinct climatic conditions and characteristic fauna and therefore the faunal distribution is not uniform everywhere (Reddy, 1992). The climatic regime of Australia, South Africa and Western South America is very much similar, but the faunal and floral population in each of these regions are different. Several species exhibit a wide range of geographic distribution but if the wide ranging species reach extinction in the intermediate territory of its range then this will lead to development of widely separated populations. Besides certain geographic regions are inhabited by unique organisms which are not found anywhere else in the world. The typical geographic distribution is called as restricted distribution. Pouched mammals (marsupials or metatherians) are confined to Australia and New Zealand. Lemurs are specifically found in the island of Madagascar. The uneven geographic distribution of animal species can be deciphered on the basis of organic evolution and certainly not on the basis of theory of special creation.

In 1876, Alfred Wallace divided earth into six principle biogeographic realms – Nearctic (North America), Palaearctic (North Asia, North Africa and Europe), Neotropical (Central and South America), Ethiopian (South Africa), Oriental (India, Sri Lanka, Malayasia, Indonesia and Philippines) and Australian (Australia, New Zealand and nearby areas).

Fig VI: Major biogeographic realms of the world

(Source: Rastogi, 2004)

Discontinuous distribution constitutes important aspect of bio-geographic evidence. It relates to the fact that closely related species inhabit widely separated places without any representative in the intermediate territory. Tapirs exist in tropical America and Malayan islands. This exhibits that these faunal species are the outcome of progressive evolution and adaptation of such animals, which have been able to reach these places in the past by migration (Reddy, 1992).

Taxonomical Evidence

Reddy (1992) defined taxonomy as the science which deals with the nomenclature and classification of individual organisms into species and the clustering of species to develop genera, families, orders, classes and kingdoms. ‘Natural affinities’ and ‘actual kinship’ (based on genetic relationship) among the organisms forms the basis for the classification. The fundamental fact that animals could be grouped and classified in order of increasing complexity is a crucial evidence of evolution. Traditionally, taxonomic classification was based upon physical similarities between species and lineages but with the arrival of genetic technologies (such as the DNA-sequencing techniques), surplus genetic evidence have emerged over the past few years that provide direct between-species comparisons of DNA sequences (Jurmain et al, 2012).

In the context of human evolution, the order primate, which belongs to a larger group – the class Mammalia (which is a part of phylum Chordata), is divided into two traditional suborders, namely, categories, Prosimii (all the prosimians: lemurs, lorises, and, customarily, the tarsiers) and Anthropoidea (all the monkeys, apes, and humans). Humans are clustered with apes and monkeys in the suborder Anthropoidea depending upon their similarities in various characteristics with apes and monkeys. The common characteristics that prevail in a species, genus, family, order or higher taxonomic rank exhibit a common ancestry but the differences amongst them indicate evolution or modifications. As the higher taxonomic rank is ramified into lower taxonomic categories, the specificity increases and relatedness decreases. With reference to primate taxonomy, Jurmain et al (2012) emphasized that the suborder distinction is more specific than the order because at the suborder level, the prosimians form a separate group which is different from all the other primates. Therefore, the above statement has biological and evolutionary relevance.

Fig VII: Classification of Homo sapiens within the order Primates

(Source link: http://media.web.britannica.com/eb-media/89/389-050-6961F221.jpg)

The natural system of classification relates to a tree like form. The initial branching of this taxonomic tree indicates the division of the whole organic world into plant and animal kingdoms followed by further ramification of these kingdoms into phyla, classes, orders and so on. The population of living species indicate the terminal twigs of the phylogenetic tree and do not indicate any direct relationship.

Summary

‘Descent with modifications’ forms the core of doctrine of organic evolution. New life forms develop from pre-existing ones through modifications. Over millions of years, the diversity of life forms on earth has witnessed drastic changes. Evolutionary modifications are basic characteristics of living organisms. The living organisms exhibit differences in size, shape, anatomical structure and functions and morphological characters but they exhibit similarities in basic and fundamental metabolic processes and chemical composition. This indicates that all living organisms have evolved from a common ancestral form.

Modern man (Homo sapiens sapiens) has evolved from non-human ancestors and it is believed that man and apes shared a common ancestor. Certain conclusive evidences or proof lay emphasis upon organic evolution and help to study the course of human evolution. These different phenomena, which yield proof and support for the course of organic evolution, include comparative morphology and anatomy, comparative embryology, biochemistry and comparative physiology, genetic analysis, palaeontology, taxonomy and biogeography.