During the Cambrian era in Earth’s history, some 550–600 million years ago, many examples of multicellular, sexually-reproducing, complex and differentiated life forms emerged for the first time. This period is known as the Cambrian explosion, and heralds an enormous leap in the complexity of organisms found on Earth. (GETTY)

What Was It Like When Life’s Complexity Exploded?

We’re a long way from the beginnings of life on Earth. Here’s the key to how we got there.

The Universe was already two-thirds of its present age by the time the Earth formed, with life emerging on our surface shortly thereafter. But for billions of years, life remained in a relatively primitive state. It took nearly a full four billion years before the Cambrian explosion came: where macroscopic, multicellular, complex organisms — including animals, plants, and fungi — became the dominant lifeforms on Earth.

As surprising as it may seem, there were really only a handful of critical developments that were necessary in order to go from single-celled, simple life to the extraordinarily diverse sets of creatures we’d recognize today. We do not know if this path is one that’s easy or hard among planets where life arises. We do not know whether complex life is common or rare. But we do know that it happened on Earth. Here’s how.

This coastline consists of quartzite Pre-cambrian rocks, many of which may have once contained evidence of the fossilized lifeforms that gave rise to modern plants, animals, fungi, and other multicellular, sexually-reproducing creatures. These rocks have undergone intensive folding over their long and ancient history, and do not display the rich evidence for complex life that later, Cambrian-era rocks do. (GETTY)

Once the first living organisms arose, our planet was filled with organisms harvesting energy and resources from the environment, metabolizing them to grow, adapt…