Читать книгу IN THE BEGINNING - Welby Thomas Cox Jr. - Страница 5

Millions of Years Since First Known Appearance (Approximate)

Microbial (procaryotic cells): 3,500

Complex (eucaryotic cells): 2,000

First multicellular animals: 670

Shell-bearing animals: 540

Vertebrates (simple fishes): 490

Amphibians: 350

Reptiles: 310

Mammals: 200

Nonhuman primates: 60

Earliest apes: 25

Australopithecine ancestors of humans: 5

Modern humans: 0.15 (150,000 years)

So many intermediate forms have been discovered between fish and amphibians, between amphibians and reptiles, between reptiles and mammals, and along the primate lines of descent that it often is difficult to identify categorically when the transition occurs from one to another particular species. Actually, nearly all fossils can be regarded as intermediates in some sense; they are life forms that come between the forms that preceded them and those that followed.

The fossil record thus provides consistent evidence of systematic change through time—of descent with modification. From this huge body of evidence, it can be predicted that no reversals will be found in future paleontological studies. That is, amphibians will not appear before fishes, nor mammals before reptiles, and no complex life will occur in the geological record before the oldest eucaryotic cells. This prediction has been upheld by the evidence that has accumulated until now: no reversals have been found.