The wide range of evidence of evolution 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 of a few soft-bodied organisms.
Comparison of the genetic sequence of organisms has revealed that organisms that are phylogenetically close have a higher degree of sequence similarity than organisms that are phylogenetically distant. Further evidence for common descent comes from genetic detritus such as pseudogenes, regions of DNA which are orthologous to a gene in a related organism, but are no longer active and appear to be undergoing a steady process of degeneration. Since metabolic processes do not leave fossils, research into the evolution of the basic cellular processes is also done largely by comparison of existing organisms. Many lineages diverged at different stages of development, so it is theoretically possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor.
Evidence from palaeontology
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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.[citation needed] There are also some trace "fossils" showing moulds, cast or imprints of some previous organisms.
As an animal dies, the organic materials gradually decay away, 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. Fossil evidence supports the theory that organisms tend to progressive increase in complexity. 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, the ages of various strata and the fossils found were roughly estimated by geologists. They did so, for instance, by estimating the time for the formation of sedimentary rock layer by layer. Today, by measuring the proportions of radioactive and stable elements in a given rock, the ages of fossils can be more precisely dated by scientists. This technique is known as radiometric dating.
Throughout the fossil record, many species which 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. Geographical regions and climatic conditions have varied throughout the Earth's history. 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 lineage that have remained morphologically (and probably also physiologically) somewhat unchanged for a very long time. Consequently, they are called "living fossils" by laymen. Examples of "living fossils" include the tuatara, the nautilus, the horseshoe crab, the coelacanth, the ginkgo and the metasequoia.
Tiktaalik roseae fish fossil
Paleontologists have found fossils of a newly-discovered species on Canada’s Ellesmere Island, located in the Nunavut Providence, that provide a key look at the evolution of fish into land animals. The fossils were found by Edward Daeschler of the Academy of Natural Sciences in Philadelphia, Neil Shubin of the University of Chicago, Farish Jenkins Jr. of Harvard and their colleagues. They found the fossils in the rocks of Ellesmere Island in 2004 after a four-year search in the area. Their findings appeared in the journal Nature on April 6, 2006.
The creature, which lived 375 million years ago during the Devonian time period, has been dubbed Tiktaalik roseae, and represents a long-sought link to the time when animals were first moving out of the primordial ocean and onto land, say scientists. The name Tiktaalik comes from a word in the Inuktitut language, meaning large, shallow water fish. The name was supplied by Inuit elders in Nunavut.
The three well-preserved specimens each have a head that looks like a crocodile and a body like a fish, and they range in length from one metre to 2.5 metres. The new animal shows both fish and animal traits, with the scales and fins of a fish, but the ribs, neck, head and appendage bones are like those of a land animal. Primitive joints and fingers were also noted. The researchers said Tiktaalik was probably a poor swimmer, but would have been able push its body off the ground, moving on land like a seal to hunt for land-based food.
Fossil of creatures having both fish and land animals features have been found before, but Tiktaalik falls into a gap between 385 million and 365 million years ago, giving researchers more details of the transition, and a critical piece of the evolutionary puzzle that directly links sea creatures to land animals. [1]
Ethiopia pre-human species fossils
The latest fossil unearthed from a human ancestral hot spot in Africa allows scientists to link together the most complete chain of human evolution so far. The 4.2 million-year-old fossil discovered in northeastern Ethiopia helps scientists fill in the gaps of how human ancestors made the giant leap from one species to another.
That's because the newest fossil, the species Australopithecus anamensis, was found in the region of the Middle Awash -- where seven other human-like species spanning nearly 6 million years and three major phases of human development were previously discovered.
"We just found the chain of evolution, the continuity through time," study co-author and Ethiopian anthropologist Berhane Asfaw said in a phone interview from Addis Ababa. "One form evolved to another. This is evidence of evolution in one place through time." [2]
Evolution of the horse
- Further information: 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 (phylogeny).
This evolutionary sequence starts with a small animal called the Hyracotherium 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; the 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; and the incisors were small, the molars having low crowns with rounded cusps covered in enamel.
The probable course of development of horses from Hyracotheium 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 0.4m to 1.5m);
- 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.
Fossilized plants found in different strata show that the marshy, wooded country in which Hyracotherium lived became gradually drier. Survival now depended on the head being in an elevated position for gaining a good view of the surrounding countryside, and on a high turn of speed for escape from predators. Hence the increase in size and the replacement of the played-out foot by the hoofed foot. The drier, harder ground would make the original splayed-out foot unnecessary for support. The changes in the teeth can be explained by assuming that the diet changed from soft vegetation to grass. A dominant genus from each geological period has been selected to show the progressive development of the horse. However, it is important to note that there is no evidence that the forms illustrated are direct descendants of each other, even though they are related.
Limitations
The fossil record is an important source for scientists when 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. These gaps are often referred to as "missing links".
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 fossil of sponges, cnidarians (e.g., corals), echinoderms (e.g., brittle stars), molluscs (e.g., snails) and arthropods (e.g., trilobites) are found. In the later Ordovician period, the first animal that really possessed the features of vertebrates (a fish) appeared. Thus few, if any, fossils of an intermediate type between invertebrates and vertebrates have been found, although likely candidates include the pikaia.
Some of the reasons for the incompleteness of fossil records are:
- In general, the probability that an organism becomes fossilized after death 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 to occur in;
- Many fossils have been destroyed by land movements and erosion;
- 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 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:
- Groups with little in common are assumed to have diverged from a common ancestor much earlier in geological history than groups which have a lot in common;
- in deciding how closely related two animals are, a comparative anatomist looks for structures which, though they may serve quite different functions in the adult, are fundamentally similar, suggesting a common origin. Such structures are described as homologous; and
- in cases where the similar structures serve different functions in adults, it may be necessary to trace their origin and embryonic development, to look for more similarities derived from a common ancestor.
When a group of organism share a homologous structure which is specialized to perform a variety of functions in order to adapt different environmental conditions and modes of life are called adaptive radiation. The gradual spreading of organisms with adaptive radiation is known as divergent evolution.
Pentadactyl limb
The pattern of limb bones called pentadactyl limb is an example of homologous structures (Fig. 5a). 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. This phenomenon is clearly shown in the forelimbs of mammals. For example:
- In the monkey, the forelimbs are much elongated to form a grasping hand for climbing and swinging among trees.
- In the pig, the first digit is lost, and the second and fifth digits are reduced. The remaining two digits are longer and stouter than the rest and bear a hoof for supporting the body.
- In the horse, the forelimbs are adapted for support and running by great elongation of the third digit bearing a hoof.
- The mole has a pair of short, spade-like forelimbs for burrowing.
- The anteater uses its enlarged third digit for tearing down ant hills and termite nests.
- In the whale, the forelimbs become flippers for steering and maintaining equilibrium during swimming.
- In the bat, the forelimbs have turned into wings for flying by great elongation of four digits, and the hook-like first digit remains free for hanging from trees.
Insect mouthparts
The basic structures are the same which include 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 (Fig. 5b):
(A) Primitive state — biting and chewing: e.g. grasshopper. Strong mandibles and maxillae for manipulating food.
(B) Ticking and biting: e.g. honey bee. Labium long to lap up nectar; mandibles chew pollen and mould wax.
(C) Sucking: e.g. butterfly. Labrum reduced; mandibles lost; maxillae long forming sucking tube.
(D) Piercing and sucking, e.g. female mosquito. Labrum and maxillae form tube; mandibles form piercing stylets; labrum grooved to hold other parts.
Analogous structures and convergent evolution
Under similar environmental conditions, fundamentally different structures in different groups of organisms may undergo modifications to serve similar functions. This phenomenon is called convergent evolution. Similar structures, physiological processes or mode of life in organisms apparently bearing no close phylogenetic links but showing adaptions to perform the same functions are described as analogous, for example:
- Wings of bats, birds and insects;
- the jointed legs of insects and vertebrates;
- tall fin of fish, whale and lobster;
- eyes of the vertebrates and cephalopod molluscs (squid and octopus). Fig. 6 illustrates difference between an inverted and non-inverted retina, the sensory cells lying beneath the nerve fibres. This results in the sensory cells being absent where the optic nerve is attached to the eye, thus creating a blind spot. The octopus eye has a non-inverted retina in which the sensory cells lie above the nerve fibres. There is therefore no blind spot in this kind of eye. Apart from this difference the two eyes are remarkably similar, an example of convergent evolution.
While previously believed to be an example of convergent evolution [L. Plate, Allgemeine Zoologie und Abstammungs-lehre, Fischer-Verlag, Jena, Germany, 1924], eye development of vertebrates and invertebrates can be linked to a common ancestor. The PAX6 gene is identified not only in squid and drosophila, but also in human and mouse and frog and zebrafish. This finding [Hill et al Nature 1991, Ton et al., Cell 1991] has re-written the biology textbooks.
Vestigial organs
Main article: Vestigial structure
A further aspect of comparative anatomy is the presence of vestigial organs. Organs that are smaller and simpler in structure than corresponding parts in the ancestral species 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 have now become unnecessary and non-functional. Examples are the vestigial hind limbs of whales, the balancers (vestigial hind wings) of flies and mosquitos, vestigial wings of flightless birds such as ostriches, and the vestigial leaves of some xerophytes (e.g. cactus) and parasitic plants (e.g. dodder).
Evidence from geographical distribution
Biologists have discovered many puzzling facts about the presence of certain species on various continents and islands (biogeography).
Continental distribution
All organisms are adapted to their environment to a greater or lesser extent. If the abiotic and biotic factors within a habitat are capable of supporting a particular species in one geographic area, then one might assume that the same species would be found in a similar habitat in a similar geographic area, e.g. in Africa and South America. This is not the case. Plant and animal species are discontinuously distributed throughout the world:
- Africa has short-tailed (Old World) monkeys, elephants, lions and giraffes.
- South America has long-tailed monkeys, pumas, jaguars and llamas.
Even greater differences can be found if Australia is taken into consideration though it occupies the same latitude as South America and Africa. Marsupials like the kangaroo can be found in Australia, but are totally absent from Africa and are only represented by the opossum in South America and the Virginia Opossum in North America:
- The echidna and duckbilled platypus, the only living representatives of primitive egg-laying mammals (monotremes), can be found only in Australia and are totally absent in the rest of the world.
- On the other hand, Australia has very few placental mammals except those that have been introduced by human beings.
Explanation
The main groups of modern mammal arose in Northern Hemisphere and subsequently migrated to three major directions:
- to South America via the land bridge in the Bering Strait and Isthmus of Panama; A large number of families of South American marsupials became extinct as a result of competition with these North American counterparts.
- to Africa via the Strait of Gibraltar; and
- to Australia via South East Asia to which it was at one time connected by land
The shallowness of the Bering Strait would have made the passage of animals between two northern continents a relative easy matter, and it explains the present-day similarity of the two faunas. But once they had got right down into the southern continents, they presumably became isolated from each other by various types of barrier.
- The submerging of the Isthmus of Panama: isolates the South American fauna
- the Mediterranean Sea and the North African desert: partially isolate the African fauna; and
- the submerging of the original connection between Australia and South East Asia: isolates the Australian fauna
Once isolated, the animals in each continent have shown adaptive radiation (Fig. 7) to evolve along their own lines.
Evidence for migration and isolation
The fossil record for the camel indicated that evolution of camels started in North America, from which they migrated across the Bering Strait into Asia and hence to Africa, and through the Isthmus of Panama into South America. Once isolated, they evolved along their own lines, giving the modern camel in Asia and Africa and llama in South America.
Continental drift
The same kinds of fossils are found from areas known to be adjacent to one another in the past but which, through the process of continental drift, are now in widely divergent geographic locations. For example, fossils of the same types of ancient amphibians, arthropods and ferns are found in South America, Africa, India, Australia and Antarctica, which can be dated to the Paleozoic Era, at which time these regions were united as a single landmass called Gondwana. [3] Sometimes the descendants of these organisms can be identified and show unmistakable similarity to each other, even though they now inhabit very different regions and climates.
Oceanic island distribution
Most small isolated islands only have native species that could have arrived by air or water; like birds, insects and turtles. The few large mammals present today were brought by human settlers in boats. Plant life on remote and recent volcanic islands like Hawaii could have arrived as airborne spores or as seeds in the droppings of birds.
Evidence from comparative embryology
Comparative embryology shows how embryos start off looking the same. As they develop, their similarities slowly decrease until they take the form of their particular class.
For example, adult vertebrates are diverse, yet their embryos are quite similar at very early stages. Fishlike structures still form in early embryos of reptiles, birds, and mammals. In fish embryos, a two-chambered heart, some veins, and parts of arteries develop and persist in adult fishes. The same structures form early in human embryos but do not persist as such in adults.
Evidence from comparative physiology and biochemistry
Evolution of widely distributed proteins and molecules
Almost all living organisms make use of DNA and ATP molecules. Furthermore, the codon codes of the DNA are the same for every organism, meaning that a piece of RNA in a bacterium codes for the same protein as in a human cell.
Cytochrome c
A classic example of biochemical evidence for evolution is the variance of the protein Cytochrome c in living cells. The variance of cytochrome c of different organisms is measured in the number of differing amino acids, each differing amino acid being a result of a base pair substitution, a mutation. If each differing amino acid is assumed to be the result of one base pair substitution, it can be calculated how long ago the two species diverged by multiplying the number of base pair substitutions by the estimated time it takes for a substituted base pair of the cytochrome c gene to be successfully passed on. For example, if the average time it takes for a base pair of the cytochrome c gene to mutate is N thousand years, the number of amino acids making up the cytochrome c protein in monkeys differ by one from that of humans, this leads us to believe that the two species diverged N million years ago.
Evidence from studies of complex iteration
"It has taken more than five decades, but the electronic computer is now powerful enough to simulate evolution" [4] assisting bioinformatics in its attempt to solve biological problems.
Computer science allows the iteration of self changing complex systems to be studied, allowing a mathematically exact understanding of the nature of the processes behind evolution; providing evidence for the hidden causes of known evolutionary events. The evolution of specific cellular mechanisms like spliceosomes that can turn the cell's genome into a vast workshop of billions of interchangeable parts that can create tools that create tools that create tools that create us can be studied for the first time in an exact way.
For example, Christoph Adami et. al. make this point in Evolution of biological complexity:
- To make a case for or against a trend in the evolution of complexity in biological evolution, complexity needs to be both rigorously defined and measurable. A recent information-theoretic (but intuitively evident) definition identifies genomic complexity with the amount of information a sequence stores about its environment. We investigate the evolution of genomic complexity in populations of digital organisms and monitor in detail the evolutionary transitions that increase complexity. We show that, because natural selection forces genomes to behave as a natural "Maxwell Demon," within a fixed environment, genomic complexity is forced to increase. [5]
For example, David J. Earl and Michael W. Deem make this point in Evolvability is a selectable trait:
- Not only has life evolved, but life has evolved to evolve. That is, correlations within protein structure have evolved, and mechanisms to manipulate these correlations have evolved in tandem. The rates at which the various events within the hierarchy of evolutionary moves occur are not random or arbitrary but are selected by Darwinian evolution. Sensibly, rapid or extreme environmental change leads to selection for greater evolvability. This selection is not forbidden by causality and is strongest on the largest-scale moves within the mutational hierarchy. Many observations within evolutionary biology, heretofore considered evolutionary happenstance or accidents, are explained by selection for evolvability. For example, the vertebrate immune system shows that the variable environment of antigens has provided selective pressure for the use of adaptable codons and low-fidelity polymerases during somatic hypermutation. A similar driving force for biased codon usage as a result of productively high mutation rates is observed in the hemagglutinin protein of influenza A. [6]
"Computer simulations of the evolution of linear sequences have demonstrated the importance of recombination of blocks of sequence rather than point mutagenesis alone. Repeated cycles of point mutagenesis, recombination, and selection should allow in vitro molecular evolution of complex sequences, such as proteins." [7] Evolutionary molecular engineering, also called directed evolution or in vitro molecular evolution involves the iterated cycle of mutation, multiplication with recombination, and selection of the fittest of individual molecules (proteins, DNA, and RNA). Natural evolution can be relived showing us possible paths from catalytic cycles based on proteins to based on RNA to based on DNA. [8] [9] [10] [11]
References
- Darwin, Charles November 24 1859. On the Origin of Species by means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. London: John Murray, Albemarle Street. 502 pages. Reprinted: Gramercy (May 22, 1995). ISBN 0517123207
- Mayr, Ernst. What Evolution Is. Basic Books (October, 2002). ISBN 0465044263
- Gigerenzer, Gerd, et al., The empire of chance: how probability changed science and everyday life (New York: Cambridge University Press, 1989).
- Williams, G.C. (1966). Adaptation and Natural Selection: A Critique of some Current Evolutionary Thought. Princeton, N.J.: Princeton University Press.
- Biological science, Oxford, 2002.
- CJ Clegg, 1999, Genetics and Evolution, John Murray. ISBN 0-7195-7552-4
- Y.K. Ho, 2004, Advanced-level Biology for Hong Kong, Manhattan Press. ISBN 962-990-635-X
External links
- 29+ Evidences for Macroevolution: The Scientific Case for Common Descent
- National Academies Evolution Resources
- Evolution by Natural Selection — An introduction to the logic of the theory of evolution by natural selection
- Evolution — Provided by PBS.
- Howstuffworks.com — How Evolution Works
- Evolution News from Genome News Network (GNN)
- National Academy Press: Teaching About Evolution and the Nature of Science
- Transitional Vertebrate Fossils FAQ