of limbs from fins in early tetrapods. Moreover, biologists have discovered that very similar sets of regulatory proteins occur in organisms as different as flies, mice, and humans, despite the many millions of years that separate these organisms from their common ancestors. The DNA evidence suggests that the basic mechanisms controlling biological form became established before or during the evolution of multicellular organisms and have been conserved with little modification ever since.
Study of all the forms of evidence discussed earlier in this booklet has led to the conclusion that humans evolved from ancestral primates. In the 19th century, the idea that humans and apes had common ancestors was a novel one, and it was hotly debated among scientists in Darwin’s time and for years after.
The Evolution of Whales, Dolphins, and Porpoises
The combination of fossil and molecular evidence enables biologists to construct much more detailed evolutionary histories than have been possible in the past. For example, recent fossil discoveries in Asia have revealed a succession of organisms that, beginning about 50 million years ago, moved from life on land first to hunt and then to live continuously in marine environments. This fossil evidence accords with recent genetic findings that whales, dolphins, and porpoises are descended from a group of terrestrial mammals known as artiodactyls, which today includes such animals as sheep, goats, and giraffes. Most recently, studies of regulatory networks in the DNA of modern porpoises have revealed the molecular changes that caused the ancestors of these organisms to lose their hind limbs and develop more streamlined bodies. All of these forms of evidence support each other and add fascinating details to the understanding of evolution.