Evidence for evolution by natural selection includes several areas of observation and inquiry including:
- Fossil evidence—Examination of ancient species reveals many that are no longer in existence or that have been dramatically altered, e.g. dinosaurs. This area of study also allows for comparison of life forms occurring before and after a given fossil and has resulted in a complex body of knowledge related to evolutionary progress.
The study of fossil evidence is tedious and frustrating by its nature. Many species do not evolve and either remain stagnant or become extinct. Most plants and animals decompose and disappear at death leaving no trace. Others are buried beyond retrieval inside layers of nearly impenetrable rock. Soft tissue does not fossilize for the most part. It is obvious that the fossil record is woefully incomplete. The total number of species that ever lived on earth is estimated to be between 17 million and 4 billion. The number is more likely than not to approach the high end of that estimation given that 10 million species live on the earth today. Only some 250,000 fossil species have been found, about 0.1-1.0% of all species that have lived. Imagine how many arks it would have taken just for the fossil species we know about, let alone the great mass that are unknown to us. (See stratigraphy)
2. Transitional forms—There has been a widely voiced contention against the theory of evolution that “missing links” or intermediate life forms between ancestral creatures and that of the presumed descendants have not been found despite long arduous efforts by zealous scientists. That is incorrect. Here are several clear-cut examples:
Ten specimens of Tiktaalik roseae, were discovered in 2004 on Canada’s Ellesmere Island by Neil Shubin and Ted Daeschler who sought for a transitional form specifically in sedimentary rock of an age (approximately 375 million years old) predicted by evolutionary theory that such a transitional animal should be found. Tiktaalik is a creature that seems to have the head of a crocodile and the body of a fish. It is a 375 million year old fossil form which was technically a fish with a jaw, scales and gills, but had unusual fins. The fins had thin ray bones suitable for paddling like fishes but they also had sturdy interior bones that enabled it to prop itself in shallow water and to use its limbs for support as most four-legged animals do to crawl out onto dry land. It had a flattened head with eyes and nostrils on top of its head which would have allowed it to live in shallow water and peer above the surface. Tiktaalik is an immediate precursor to the amphibians that first left a predominately aquatic life to live on land. It had a neck like early amphibians. Its skull, neck, ribs, and pectoral fin are like the earliest tetrapods. The sturdy pectoral fin has the precursors of the humerus, radius, and ulna of tetrapod arms with a bendable elbow and an extendible proto-wrist. It is, therefore, a transition form between ancient fishes and their descendants, all the four-legged vertebrates including amphibians, dinosaurs, birds, and mammals like humans. Eusthenopteron and Acanthostega were other, less definitive examples.
The primitive aquatic nymph was found in fossils dated 300 MYA. This simple creature had a significant feature—wing-like structures on all thoracic and abdominal segments. The wing-like nature of those structures is reflected in the pattern of canals on the appendages, which is altogether similar to the pattern of veins in winged insects. However, this fossil is of an aquatic animal, and the structures are not wings, but gills similar to those found on the aquatic nymph larval stages of dragonflies and mayflies. The most likely scenario, therefore, for the origin of wings is that the wings of adult terrestrial insects evolved in animals that also had gills in their larval stages, another example of ontology recapitulating phylogeny and the tinkering of old genes to make new structures and functions.
A transitional ant that lived 92 MYA was found preserved in amber. The earliest known snake–one that lived 90 MYA–had a small pelvic girdle and reduced hind legs. The earliest known chordate–an animal that lived 530 MYA–called Haikouella lanceolata, was found in China. This inch long, eel-like fish had a frilly dorsal fin. The interesting thing about this creature was that it had a head and a circulatory system. The fascinating thing and that which makes Haikouella a crucial find, is that it had a brain and a cartilaginous bar along its back—the first known notochord. It was a true chordate, and chordates gave rise to all vertebrates, which eventually gave rise to mammals, which ultimately gave rise to human beings.
Genetic evidence clearly links the modern hippopotamus and modern whales evolutionarily. 60 MYA, there were no fossils of modern whales and none appeared until 30 million years later. However, 10 million years later than the period where there was no indication that deep sea mammals lived, fossil evidence began to appear. Intermediates include Pakicetus (52 MYA) which had simple teeth and whalelike ears, Ambulocetus (50 MYA) “the walking whale” which had reduced but robust limbs with hooves and could waddle on land like a seal, the partially aquatic Indohyus (48 MYA) with special features of teeth and ears found only in whales and their ancestors. Rodhocetus (47 MYA) had a more elongated skull and nostrils further back on its skull and was still more aquatic having a reduced pelvis and smaller limbs so that walking on land would have been awkward and difficult but was a good swimmer. Next came Basilosaurus and Dorudun (40 MYA) with short necks and blowholes on top of their skulls The next intermediate or transitional form clearly accepted as a “link” is Aetiocetus.(25 MYA). Aetiocetus had its nostrils at the middle of its skull unlike Pakicetus with its nostrils at the front of its skull. The Beluga whale’s (23-5.33 MYA with a fossil dating to 7-10 KYA found in the US) nostrils are located at the top of its skull, and it has a highly flexible neck. The Beluga is the modern counterpart of Aetiocetus. As they now exist, whales are stretched out land animals whose forelimbs have become paddles, and whose nostrils have moved atop their heads. Evolution tinkered with the parts of land creatures and ended up with whales; whales did not just appear de novo in completed form.
An ancient slope-faced ape, Pierolapithecus catalaunicus, informally nicknamed Pau, may be the last, or at least the latest found, common ancestor of the great ape family—gorillas, chimpanzees, orangutans, and humans. 12.5-13 million year old fossils were found in northeastern Spain including complete remains totaling 83 bone fragments. Pau weighed between 66 and 77 pounds and stood upright almost four feet tall. Evolutionary reconstructions indicate that great apes branched off from the larger primate family tree between 11-12 MYA, but there has been a relative void of fossils from that period; so, this find is the first good evidence from that transitional era. Pau had a broad sloping face, wide rib cage and a flexible wrist suitable for tree climbing, characteristics of apes and humans. He also had small hands with straight fingers that were monkey-like similar to more ancient primates. He was transitional in that, although he could live in trees and hang there, he was unable to swing from branch to branch like later apes, an adaptation that probably evolved independently in chimpanzees and orangutans long after these two groups diverged. Man came much later from the more immediate pre-chimpanzee line.
Reptiles preceded birds. The Maniratoa, reptile progenitors of the pterodactyl, Archaeopteryx, had full teeth, a furucula (wish-bone), a flat sternum, aiglon fossil belly ribs, wings, three curved claws on its wings, wing feathers, tail feathers, and reduced fingers—suggestive as characteristics of modern birds. Archaeopteryx lithographica, which lived about 150 MYA, is considered by many to be the first bird having bird feathers including tail feathers and reptile teeth, and was the size of a European magpie, but had enough reptilian characteristics for it to be considered more likely an intermediate “missing-link” form. Researchers found a 120 MYA sparrow sized pterodactyl in China’s Liaoning province, a region that was forested at the time the reptile, called Nemicolopterus crypticus, lived. Unlike Archaeopteryx, this small reptile had curved claws and attachment sites on bones for muscles that would have facilitated clinging to tree branches. Also, unlike most pterodactyls, it lacked teeth and probably ate insects. Modern birds appeared about 100 MYA during the Mesozoic Age with flat claws that facilitated terrestrial rather than arboreal living.
Another example is found in the evolution of horse feet which appeared in the fossil record. 55 MYA. Hyracotherium had four toes; more recently Miohippus and Parphippus had three; and more than 6 MYA came a primitive single toed animal, Pliohippus. 4 MYA Equus, the direct progenitor of the modern horse with its well developed single toe, appeared. This fossil record is more complex than that of Aetiocetus because of a multi-branched horse evolutionary tree, and the line given above is not a direct one. In each of these cases it is reasonable to presume that paleontologists will find still more intermediate forms, but the effort and expense required is immense given the vastness of the earth, the difficulty of finding and extracting fossil material from amidst mountains of rock, and the fragility of the creatures that resulted in permanent loss of many individuals and whole genuses, and the staggering costs involved in such painstaking and time consuming research.
Finally, consider Australopithecus afarensis, a recognizable forerunner of humans. The diminutive creature was a mix of human and ape-like characteristics. It was small with a chest having a chimpanzee-like funnel shape rather than the human-like barrel shape. It had long arms and hooked fingers adapted for hanging from branches, but its mid-skull foramen magnum, pelvis, and stiff feet which were definitely human-like in that they were adapted for a life of standing and walking on two legs including fast locomotion, unlike the apes with their posteriorly positioned foramen magnum, short bent legs and flexible feet that were (and still are) so remarkably adapted for tree living and so clumsy when running on flat ground. continued…