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.

Jim Secord, Director of the Darwin Correspondence Project, University of Cambridge, explains the traits Darwin thought to be fundamental to humans.

About Jim Secord "I’m the Director of the Darwin Correspondence Project in the University Library at Cambridge University.

My research is on public debates about science in the 18th and 19th centuries. I’ve written particularly about Victorian evolution and debates about the problem of species and where we come from. I’ve also written about the reception of evolutionary works before Darwin published his Origin of Species."

What it means to be human is a crucial question for Darwin. You get an interesting impression of this from his writings. If you read On the Origin of Species, for example, there’s hardly anything in it about human beings. There is one sentence where he says light will be thrown on the origin of man and its history. And there are a few other examples that involve people.

What needs to be emphasized is that the core idea of On the Origin of Species, evolution by natural selection, comes from thinking about humans and what it means to be human. It comes from the work of political economy by Thomas Malthus, An Essay on the Principle of Population. When Darwin was thinking about why some individuals survive in the struggle for existence and others don’t make it, he was thinking about people; he was thinking about you and me.

I think it’s really important to realise that throughout Darwin’s theoretical thought, this question about humanity and the idea of what it means to have a mind and be part of the natural world at the same time – this is the core of the question he’s trying to answer. We can see this in his later writings when he publishes The Descent of Man, and Selection in Relationship to Sex in 1871. This is a two-volume work, which argues in the first part that humans come from lower animals. In making this argument, Darwin shows how a whole range of different characteristics – our morals, our belief in God, our love of music – are in various ways present within lower animals.

Emanuele Coccia, Associate Professor of Historical Anthropology at EHESS, discusses metamorphosis.

About Emanuele Coccia "I am Professor at the École des Hautes Études en Sciences Sociales in Paris.

I’m working on art, fashion and ecology. I published The Life of Plants four years ago, and this year I published a book called Metamorphoses."

Key Points

• Metamorphosis shows that life cannot be reduced to a single anatomical or moral identity. • Metamorphosis is the relationship between all individuals within a species as well as the relationship between species. • Living beings come from previously living beings; every life is more ancient than the body carrying it.

Metamorphosis is interesting because it tells us about what life is in general. For example, take the most evident phenomenon of metamorphosis: insect metamorphosis. In this case, metamorphosis shows that there is life between two bodies that have nothing in common from an anatomical, ecological or ethological viewpoint.

A caterpillar and a butterfly do not share the same body. They do not share the same face. They are totally different. Moreover, they do not share the same ethos or moral identity. On the one hand, you have a caterpillar, which is a life form occupied by the question of nutrition. For a caterpillar, the world is a huge McDonald’s where it can simply eat. On the other hand, you have the butterfly, which is a life form whose main concern is having sex.