Buzz Baum, Cell Biologist at the MRC Laboratory of Molecular Biology in Cambridge, explains the beginnings of life on Earth.

Key Points

• Darwin hypothesised that all living organisms are branches on a tree and that there is one single trunk to life on Earth. • Two partners gave rise to all complex cells: bacteria and archaea. We are all composite organisms – a mixture of bacterial genes and archaea organisms. • Many aspects of archaeal biology are very similar to our biology. So, although we’re separated by billions of years, if you look closely, these tiny cells share a lot of biology with us.

The Tree of Life Like many people, I’ve always been interested in nature, in the beautiful creatures around us, when we go walking in a forest or when we see all these different creatures. For most of human history, people didn’t really ask whether there were common ancestors. The first person to wonder whether, despite all the diversity of life on Earth, there are any commonalities, some sort of common relationships, was Charles Darwin.

In his notebooks, he had a picture of a tree, and he used this metaphor that all living organisms are branches on the tree and they all join up and there’s one trunk. He imagined in his sketchbooks that there was this single trunk to life on Earth. And we now know that his intuition was correct. Once upon a time, there was one organism and that organism gave rise to everything on Earth. In a way, you could look at all life on Earth as one colony. So, just as each of us starts as a single cell and gives rise to a whole body, the whole of life on Earth began as one cell, and is descended from a single cell, and that cell grew and divided, and grew and divided, and gave rise to the whole of life on Earth.