The moment we became human

In the film of Stanley Kubrick 2001: A Space Odyssey, monkeys become human in contact with a mysterious black monolith planted in the desert. The idea has remained
In the film of Stanley Kubrick 2001: A Space Odyssey, monkeys become human in contact with a mysterious black monolith planted in the desert. The idea has remained

About 500,000 years ago, an error in the DNA replication of our ancestors produced a mutation. This would be at the origin of the modern brain.

It is often said that the Scottish biologist Alexander Fleming invented antibiotics by mistake. That is true. He had forgotten culture dishes infected with microscopic fungi, and noted that around these fungi the bacteria died. He later identified the substance produced by molds, a substance with a bactericidal effect. Penicillin. The first of antibiotics.

The greatest progress sometimes results from an error.

From a fortuitous accident with incalculable consequences. Take, for example, the human brain. This organ is now regarded by biologists as the most complex object in the whole universe. In figures: about 100 billion neurons and one million billion synapses. It can learn and read, write, speak several languages, play a musical instrument, decipher the emotions of others, make plans for several years, read Shakespeare and enjoy a great Bordeaux. How did we get there ?

In search of missing genes

Let us go back 500,000 years. Our ancestors Homo erectus then travel the planet with brains about twice as small. They are not Homo sapiens yet. They lack a trigger to master abstraction and complex notions. And then, this click happens.

Paleogenetics, specialists in prehistoric DNA, have just discovered how. They found the answer in the genome of our distant ancestors. The Swedish researcher Svante Paabo was the pioneer of this research. His team has just discovered that a fortuitous mutation, which appeared about 500 000 years ago in the genome of hominids, has changed everything for our brain.

This mutation is the simplest one can imagine in genetics: it concerns the letters A, C, T, G which form the double helix of DNA, and which are called nitrogenous bases. A C was replaced in error (the cells sometimes make DNA replication errors) by a G. But this substitution had an unexpected effect: the gene that was there contained, from that moment, a Trio of letters GTA (and not CTA), and this trio of letters is interpreted by the cells of the body as a cutoff signal: at this point, the gene is cut by enzymes and then shortened by a process called splicing. As a result, the gene concerned was amputated with 55 nitrogen bases.

The mutation that changed everything

The shorter gene we have today has very different functions than those at work in our ancestors. In the past, it regulated the growth of actin filaments in neurons, the proteins that form the internal skeleton of cells. The shortened version no longer fulfills this function, but stimulates the division of neurons from parent cells, called progenitors, in certain crucial areas of the brain, the ventricles. In other words, the mutated gene we inherited causes our brain to produce many more neurons. The progenitor cells divide at an increased rate, giving rise to young neurons which then migrate from the ventricles to the periphery of the brain, participating in the growth of the cortex; Which, as it expands, is forced to wrinkle to „hold“ in a compact manner inside the cranial box, giving this folded aspect characteristic of the human brain.

A sudden proliferation of neurons …

The Paabo team transferred the mutated version of the gene to mice, and found that their brain produced neurons at an increased rate, well above the usual rate. Hence, it was probably only a point change on a single link in DNA, to transform a brain of erectus into the brain of Homo sapiens. This change was probably a mistake, a failure in the mechanisms of DNA replication in an anonymous Homo erectus that will probably never be known if he has lived in West Africa, Asia or Europe, and who is The father of men with modern brains. But this error proved prodigiously fruitful: in fact, the descendants of this first mutant probably benefited from such advantages of abstraction and conceptualization that they quickly spread, transmitting the mutated gene to future generations, and up to The whole of humanity. Inventing wheel, writing and antibiotics. But also, it is true, the atomic bomb, the kalashnikov and global warming. How an error is never without consequences.