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 likely bear a strong resemblance to those early primates. (GETTY)

What Was It Like When Mammals Evolved And Rose To Prominence?

The history of life on Earth took many meandering turns before it gave rise to us.

When Earth first formed, all the raw ingredients for life — atoms, molecules, a potentially habitable planet at the right distance from its star — were already in place. While life itself arose relatively quickly, 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 can have 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 sequences and physical traits from their parents.

All of this, in tandem, led to the Cambrian explosion some 550–600 million years ago. But the rise of warm-blooded mammals to prominence would take nearly another half-a-billion years.