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11.2 Evidence of Universal Common Descent

11.2.1 Common Biochemistry and Genetic Code

All known forms of life are based on the same fundamental biochemical organisation: genetic information encoded in DNA, transcribed into RNA through the effect of protein and RNA-enzymes, then translated into proteins by (highly similar) ribosomes, with ATP, NADH and others as energy sources, etc. Furthermore, the genetic code (the “translation table” according to which DNA information is translated into proteins) is nearly identical for all known life forms, from bacteria to humans. The universality of this code is generally regarded by biologists as definitive evidence in favour of the theory of universal common descent. Analysis of the small differences in the genetic code has also provided support for universal common descent. Selectively Neutral Similarities

Similarities which have no relevance to evolution and therefore cannot be explained by convergence, tend to be very compelling support for the universal common descent theory.

Such evidence has come from two domains:

– Amino acid sequences

– DNA sequences.

Proteins with the same three-dimensional structure need not have identical amino acid sequences. In certain cases, there are several codons (DNA triplets) that code for the same amino acid. Thus, if two species use the same codon at the same place to specify an amino acid that can be represented by more than one codon, that is evidence for a recent common ancestor. Other Similarities

The universality of many aspects of cellular life is often pointed to as supportive evidence to the more compelling evidence listed above. These similarities include the energy carrier ATP, and the fact that all amino acids found in proteins are left-handed. It is possible that these similarities resulted because of the laws of physics and chemistry, rather than universal common descent and therefore resulted in convergent evolution.

A phylogenetic tree based on rRNA genes.

Another important piece of evidence is that it is possible to construct detailed phylogenetic trees (that is, “genealogic trees” of species) mapping out the proposed divisions and common ancestors of all living species. Traditionally, these trees have been built using morphological methods (based on appearance, embryology, etc). Recently, it has been possible to construct these trees using molecular data (based on similarities and differences between genetic and protein sequences). All these methods produce essentially similar results, despite the fact that most genetic variation has no influence over external morphology. The fact that phylogenetic trees based on different types of information agree with each other is strong evidence of a real underlying phylogeny -that is, common descent.

11.2.2 Illustrations of Common Descent Artificial Selection

Artificial selection offers remarkable examples of the amount of diversity that can exist between individuals sharing a late common ancestor. To perform artificial selection, one begins with a particular species and then, at every generation, only allows certain individuals to reproduce, based on the degree to which they exhibit certain desirable characteristics. In time, it is expected that these characteristics become increasingly well-developed in successive generations. Dog Breeding

An obvious example of the power of artificial selection is the diversity found in various breed in domesticated dogs. The various breeds of dogs all share common ancestry (being all ultimately descended from wolves) but were domesticated by humans and then selectively bred in order to enhance various features such as coat colour and length or body size. Wild Cabbage

Early farmers cultivated many popular vegetables from the Brassica oleracea (common name wild cabbage) by artificially selecting for certain attributes. Common vegetables such as cabbage, kale, broccoli, cauliflower, kohlrabi and Brussels sprouts are all descendants of the wild cabbage plant. Darwin’s Finches

While in the Galápagos Islands, Darwin observed 13 species of finches that are closely related and differ most markedly in the shape of their beaks. The beak of each species is suited to the food available in its particular environment, suggesting that beak shapes evolved by natural selection. Large beaks were found on the islands where the primary source of food for the finches is nuts and therefore the large beaks allowed the birds to be better equipped for opening them. Slender beaks were found on the finches which found insects to be the best source of food on the island they inhabited.

11.2.3 Evidence of Common Descent

The wide range of evidence of common descent of living things strongly indicates the occurrence of evolution and provides a wealth of information on the natural processes by which the variety of life on Earth developed.

Fossils are important for estimating when various lineages developed. As fossilization is an uncommon occurrence, usually requiring hard body parts and death near a site where sediments are being deposited, the fossil record only provides sparse and intermittent information about the evolution of life. Evidence of organisms prior to the development of hard body parts such as shells, bones and teeth is especially scarce, but exists in the form of ancient microfossils, as well as impressions of various soft-bodied organisms. Evidence from Genetics

While on board HMS Beagle, Charles Darwin collected numerous specimens, many new to science, which supported his later theory of evolution by natural selection.

Although it has only recently become available, the best evidence for common descent comes from the study of gene sequences. Comparative sequence analysis examines the relationship between the DNA sequences of different species, producing several lines of evidence that confirm Darwin’s original hypothesis of common descent. If the hypothesis of common descent is true, then species that share a common ancestor will have inherited that ancestor’s DNA sequence. Notably they will have inherited mutations unique to that ancestor. More closely-related species will have a greater fraction of identical sequence and will have shared substitutions when compared to more distantly-related species.

A deeper understanding of developmental biology shows that common morphology is the product of shared genetic elements. For example, although camera-like eyes are believed to have evolved independently on many separate occasions, they share a common set of light-sensing proteins (opsins), suggesting a common point of origin for all sighted creatures. Evidence from Paleontology

When organisms die, they often decompose rapidly or are consumed by scavengers, leaving no permanent evidences of their existence. However, occasionally, some organisms are preserved. The remains or traces of organisms from a past geologic age embedded in rocks by natural processes are called fossils. They are extremely important for understanding the evolutionary history of life on Earth, as they provide direct evidence of evolution and detailed information on the ancestry of organisms. Paleontology is the study of past life based on fossil records and their relations to different geologic time periods.

For fossilization to take place, the traces and remains of organisms must be quickly buried so that weathering and decomposition do not occur. Skeletal structures or other hard parts of the organisms are the most commonly occurring form of fossilized remains. There are also some trace “fossils” showing moulds, cast or imprints of some previous organisms.

As an animal dies, the organic materials gradually decay, such that the bones become porous. If the animal is subsequently buried in mud, mineral salts will infiltrate into the bones and gradually fill up the pores. The bones will harden into stones and be preserved as fossils. This process is known as petrification. If dead animals are covered by wind-blown sand, and if the sand is subsequently turned into mud by heavy rain or floods, the same process of mineral infiltration may occur. Apart from petrification, the dead bodies of organisms may be well preserved in ice, in hardened resin of coniferous trees (amber), in tar, or in anaerobic, acidic peat. Fossilization can sometimes be a trace, an impression of a form. Examples include leaves and footprints, the fossils of which are made in layers that then harden. Fossil Records

It is possible to find out how a particular group of organisms evolved by arranging its fossil records in a chronological sequence. Such a sequence can be determined because fossils are mainly found in sedimentary rock. Sedimentary rock is formed by layers of silt or mud on top of each other; thus, the resulting rock contains a series of horizontal layers, or strata. Each layer contains fossils which are typical for a specific time period during which they were made. The lowest strata contain the oldest rock and the earliest fossils, while the highest strata contain the youngest rock and more recent fossils.

A succession of animals and plants can also be seen from fossil records. By studying the number and complexity of different fossils at different stratigraphic levels, it has been shown that older fossil-bearing rocks contain fewer types of fossilized organisms, and they all have a simpler structure, whereas younger rocks contain a greater variety of fossils, often with increasingly complex structures.

In the past, geologists could only roughly estimate the ages of various strata and the fossils found. They did so, for instance, by estimating the time for the formation of sedimentary rock layer by layer. Today, radiometric dating measurements are accurate.

Throughout the fossil record, many species that appear at an early stratigraphic level disappear at a later level. This is interpreted in evolutionary terms as indicating the times at which species originated and became extinct. Since organisms are adapted to particular environments, the constantly changing conditions favoured species which adapted to new environments through the mechanism of natural selection.

According to fossil records, some modern species of plants and animals are found to be almost identical to the species that lived in ancient geological ages. They are existing species of ancient lineages that have remained morphologically (and probably also physiologically) somewhat unchanged for a very long time. Extent of the Fossil Record

Despite the relative rarity of suitable conditions for fossilization, approximately 250,000 fossil species are known. The number of individual fossils this represents varies greatly from species to species, but many millions of fossils have been recovered. Many more fossils are still in the ground, in various geological formations known to contain a high fossil density, allowing estimates of the total fossil content of the formation to be made.

While the fossils cannot undoubtedly prove common descent, they are highly suggestive of it if they show two patterns:

  • Older forms are simpler than newer forms;
  • The number of species increases with time.

The fossil record meets the first criterion. Among the earliest mammalian fossils, there are no specialized mammals like whales, but we do find fossils of whale-like terrestrial mammals that possessed underdeveloped legs. The second criterion poses a sort of impasse between evolutionary scientists who claim their findings to be incomplete yet compelling and creationists who bemoan them as severely lacking.

Anti-evolutionists interpret this scarcity as a weakness in the theories of common ancestry and evolution. Some scientists claim that only the minimal availability of fossils of transitional forms points to periods of very rapid evolution interrupted by much longer periods of preservation of form. According to them “the scarcity of transitional fossils has implications for certain theories of evolutionary mechanisms and rates of evolution” but not for the evidence for common ancestry. Evolution of the Horse

Due to an almost-complete fossil record found in North American sedimentary deposits from the early Eocene to the present, the horse provides one of the best examples of evolutionary history.

Their evolutionary sequence starts with a small animal called Hyracotherium (commonly referred to as Eohippus) which lived in North America about 54 million years ago, then spread across to Europe and Asia. Fossil remains of Hyracotherium show it to have differed from the modern horse in three important respects:

  • It was a small animal (the size of a fox), lightly built and adapted for running.
  • He limbs were short and slender, and the feet elongated so that the digits were almost vertical, with four digits in the forelimbs and three digits in the hindlimbs.
  • The incisors were small, the molars having low crowns with rounded cusps covered in enamel.

The probable course of development of horses from Hyracotherium to Equus (the modern horse) involved at least 12 genera and several hundred species. The major trends seen in the development of the horse to changing environmental conditions may be summarized as follows:

  • Increase in size (from 0.4 m to 1.5 m);
  • Lengthening of limbs and feet;
  • Reduction of lateral digits;
  • Increase in length and thickness of the third digit;
  • Increase in width of incisors;
  • Replacement of premolars by molars; and
  • Increases in tooth length, crown height of molars. Limitations

The fossil record is an important source for tracing the evolutionary history of organisms. However, because of limitations inherent in the record, there are not fine scales of intermediate forms between related groups of species. This lack of continuous fossils in the record is a major limitation in tracing the descent of biological groups. Furthermore, there are also much larger gaps between major evolutionary lineages.

There is a gap of about 100 million years between the early Cambrian period and the later Ordovician period. The early Cambrian period was the period from which numerous fossils of sponges, cnidarians (e.g., jellyfish), echinoderms (e.g., eocrinoids), molluscs (e.g., snails) and arthropods (e.g., trilobites) are found. The first animal that possessed the typical features of vertebrates, the Arandaspis, was dated to have existed in the later Ordovician period. Thus few, if any, fossils of an intermediate type between invertebrates and vertebrates have been found, although likely candidates include the Burgess Shale animal, Pikaia gracilens, and its Maotianshan shales relatives, Myllokunmingia, Yunnanozoon, Haikouella lanceolata, and Haikouichthys.

Some of the reasons for the incompleteness of fossil records are:

  • In general, the probability that an organism becomes fossilized is very low;
  • Some species or groups are less likely to become fossils because they are soft-bodied;
  • Some species or groups are less likely to become fossils because they live (and die) in conditions that are not favourable for fossilization;
  • Many fossils have been destroyed through erosion and tectonic movements;
  • Some fossil remains are complete, but most are fragmentary;
  • Some evolutionary change occurs in populations at the limits of a species’ ecological range, and as these populations are likely to be small, the probability of fossilization is lower (see punctuated equilibrium);
  • Similarly, when environmental conditions change, the population of a species is likely to be greatly reduced, such that any evolutionary change induced by these new conditions is less likely to be fossilized;
  • Most fossils convey information about external form, but little about how the organism functioned;
  • Using present-day biodiversity as a guide, this suggests that the fossils unearthed represent only a small fraction of the large number of species of organisms that lived in the past. Evidence from Comparative Anatomy

Comparative study of the anatomy of groups of animals or plants reveals that certain structural features are basically similar. For example, the basic structure of all flowers consists of sepals, petals, stigma, style and ovary; yet the size, colour, number of parts and specific structure are different for each individual species. Homologous Structures and Divergent (adaptive) Evolution

If widely separated groups of organisms are originated from a common ancestry, they are expected to have certain basic features in common. The degree of resemblance between two organisms should indicate how closely related they are in evolution. Pentadactyl Limb

The pattern of limb bones called pentadactyl limb is an example of homologous structures. It is found in all classes of tetrapods (i.e. from amphibians to mammals). It can even be traced back to the fins of certain fossil fishes from which the first amphibians are thought to have evolved. The limb has a single proximal bone (humerus), two distal bones (radius and ulna), a series of carpals (wrist bones), followed by five series of metacarpals (palm bones) and phalanges (digits). Throughout the tetrapods, the fundamental structures of pentadactyl limbs are the same, indicating that they originated from a common ancestor. But in the course of evolution, these fundamental structures have been modified. They have become superficially different and unrelated structures to serve different functions in adaptation to different environments and modes of life. Insect Mouthparts

The basic structures are the same, including a labrum (upper lip), a pair of mandibles, a hypopharynx (floor of mouth), a pair of maxillae, and a labium. These structures are enlarged and modified; others are reduced and lost. The modifications enable the insects to exploit a variety of food materials. Other Arthropod Appendages

Insect mouthparts and antennae are considered homologues of insect legs. Parallel developments are seen in some arachnids: The anterior pair of legs may be modified as analogues of antennae, particularly in whip scorpions, which walk on six legs. These developments provide support for the theory that complex modifications often arise by duplication of components, with the duplicates modified in different directions. Vestigial Structures and Embryonic Development

The strongest direct evidence for common descent comes from vestigial structures and embryonic development. Rudimentary body parts are called vestigial organs. They are usually degenerated or underdeveloped. The existence of vestigial organs can be explained in terms of changes in the environment or modes of life of the species. Those organs are thought to be functional in the ancestral species but are now either non-functional or repurposed. Evidence from Geographical Distribution

Data about the presence or absence of species on various continents and islands (biogeography) can provide evidence of common descent and shed light on patterns of speciation.