- Comparative Anatomy
- Comparative Embryology
- Palaeontology
- Dating Methods
- Geographical Distribution of Species
- Molecular Biology
The understanding of human evolution is based on the interpretation of comparative anatomy and embryology, palaeontology, dating methods, geographical distribution of species and invisible molecular structure that have changed and modified along with the time. There are numerous evidences which support the occurrence and understanding of human evolution which in turn are also used as crucial methods to study human evolutionary process; some of the important methods and approaches of studying are given:
1. Comparative Anatomy
The method comparative anatomy deals with the comparative study of the body structures of different animal species including humans to understand the course of evolution. Several anatomical and morphological structures present in related animal species provide an important means to trace the evolutionary lines. These anatomical structures include:
- Homology and Analogy: Similarities in the anatomical structure of different animals having different functions is referred to as homology and structures are termed as homologous structures. Contrary to it, anatomically dissimilar animals showing similarity in their functions because of adaption along similar lines are defined as analogy and structures are called analogous structures. For example: the structure of feet of bat, forearms of horse and forearm of man are homologous structure whereas flying organs of butterfly, aves and bat present examples of analogous structure.
- Vestigial Organs: These organs bear testimony regarding the evolution of animals. Vestigial organs are remains of developed organs that have become useless or unessential after evolution. Humans and other animals show a number of instances of vestigial organs. For example: The vermiform appendix in man is a vestigial organ. Appendix is found in developed form in other primates which helps in the digestion of coarse food materials.
- Adaptive modification: Adaptive modifications in the organs of animals belonging to same class support evidences for evolution. For example, the forearms of animals belonging to class mammalia have assumed different forms according to functions in different times (Pandey, 2010).
2. Comparative Embryology
Embryological studies (individual development in earlier phases) also suggest the path of evolution. Ernest Haeckel (1866) postulated biogenetic law or recapitulation theory as he observed the generalised developmental pattern between the embryos of different animal groups. On the basis of this law attempts were made to understand that embryological changes in all multicellular animals exhibit similarity. In early stage, embryo of fish, salamander, birds and man exhibit similarity to a great extent which indicates that these animals would have evolved from a common ancestor.
3. Paleontology
Paleontological methods (study of fossils) offer important evidences to study evolution. When fossils are arranged chronologically, they present strong proof of stages through which the evolution of animals had taken place. Entire geological time scale has been formulated on the basis of fossils discovered from different stratum of rocks. Fossils have been helpful not only in establishing geological time scale, but they have also helped in solving the problem of missing link (Pandey, 2010).
4. Dating Methods
The determination of age of fossils is essential to elucidate their relationship in order to understand the path of evolution. The advanced methods used in stratigraphy and radiochemistry made it possible to establish both relative and absolute dates for many groups of fossils. Relative dating method is based on the thorough knowledge of stratigraphy which is the study of the layers or strata which makes up parts of the earth’s crust. The determination of relative age of fossils in a section of excavation is comparatively easier than fossils obtained from different sites some distance apart. In latter case, the stratigraphic needs to correlate the sequence in different sites to determine the age of one fossil in relation to other, which may introduce considerable uncertainties. Absolute dating depends on being able to determine the age in years of certain geological deposits which may contain fossils or more often underly or cover bearing strata. The techniques have been developed as a result of the discovery that certain naturally occurring radioactive elements decay at constant, known and measurable rates into other known elements. Radioactive potassium (K40) and radioactive carbon (C14) are two such elements that decay into argon and nitrogen respectively. These techniques can be used both directly and indirectly to date fossils in a number of ways and forms an essential basis for the construction of a reliable phylogenetic lineage (Campbell, 1967)
5. Geographical Distribution of Species
Another strong evidence for evolution is provided by geographical distribution of species. Geographical distribution is one of the causes that lead differences in the biological structure of animals. Due to geographical isolation many related species isolate and get adapted to the changed environment. But in spite of local changes and modifications, they exhibit ‘similarities’ which provides evidences for evolution.
6. Molecular Biology
Researches in the field of molecular biology present many evidences in support of evolution. Important molecular biological methods to understand evolution include:
- Amino-acid sequences in Proteins: Evolution of amino acid sequences in different proteins is helpful in understanding the process of evolution. For example, in mammals, difference of one amino-acid in a protein named haemoglobin shows time distance of 70 lakhs from other animals. Some other proteins like insulin, cytochrome etc. show more distance of time. From the distance of time period the rate of evolution can be predicted.
- Nucleotide sequence in Protein: Nucleic acids like DNA and RNA possess nucleotides. By studying the nucleotide sequences of these nucleic acids, evolutionary processes can be understood. In this method, hybridisation of DNA in vitro of two animals is done and difference is calculated on the basis of thermal stability (Pandey, 2010).
Furthermore, the comparison of mitochondrial genome of animals is an important method to examine the tempo and mode of molecular evolution. Mitochondria are transmitted along only female lineages and mtDNA is genetic haploid, the effective size of a population of mtDNA is a quarter of that of the corresponding autosomes. The mutation rate of the mitochondrial genome is about ten times higher than that of nuclear DNA which provides an abundance of polymorphic sites to study the rate of evolution (Cavalli-Sforza and Feldman, 2003). Cann, Stoneking & Wilson (1987) derived two important conclusions from the analysis of mtDNA. These conclusions are:
- the first major separation in the evolutionary tree of modern human was between Africans and non-Africans and
- the time back to the most common recent ancestor of modern human mtDNA was 190,000 years (Cavalli-Sforza & Feldman, 2003).