During the Cambrian explosion, some 550–600 million years ago, the first complex, differentiated, macroscopic, multicellular, sexually-reproducing animals came to dominate the oceans. Over the next half a billion years, evolution would take life in many different directions. By the time the asteroid eliminating the dinosaurs arrived, 65 million years ago, mammals had diversified in a number of directions, with the earliest primates splitting off just before that great event. Modern lemurs, shown here, likely bear a strong resemblance to those early primates. (Credit: Bas Czerwinski/AFP/Getty Images)
Although mammals may be the dominant form of life today, we’re relative newcomers on planet Earth. Here’s our place in natural history.
The evolution of life on our planet didn’t take an inevitable course to bring human beings about, but rather appears to have led to us through a series of remarkable but not necessarily mandatory events. When planet Earth first formed, all the raw ingredients for life to emerge — atoms, molecules, a potentially habitable planet at the right distance from its star — were serendipitously in place. While life itself arose relatively quickly (within the first few hundred million years) on Earth, it took billions of years for that life to become complex, differentiated, and macroscopic. The four key developments that took us there were:
horizontal gene transfer, enabling an organism to gain useful genetic sequences from other species,
eukaryotic cells, whereby individual cells come to possess their own specialized organelles, enabling the performance of unique functions,
multicellularity, allowing further specialization and differentiation,
and sexual reproduction, enabling slowly-reproducing organisms to have dramatically different DNA…
