Lucky you! Accidents of evolution that made us human

New Scientist, 06 June 2012 by Clare Wilson:

EARTH, several million years ago. A cosmic ray blasts into the atmosphere at close to the speed of light. It collides with an oxygen atom, generating a shower of energetic particles, one of which knocks into a DNA molecule within a living creature.

That DNA molecule happens to reside in a developing egg cell within an ape-like animal living in Africa. The DNA is altered by the collision - mutated - and the resulting offspring is slightly different from its mother.

The mutation gives the offspring an advantage over its peers in the competition for food and mates, and so, as the generations pass, it is carried by more and more of the population. Eventually it is present in nearly everyone, and so the altered version of the DNA should really no longer be called a mutation - it's just one of the regular 23,000 or so genes that make up the human genome.

While cosmic rays are thought to be one source of mutations, DNA-copying errors during egg and sperm production may be a more common cause. Whatever their origins, these evolutionary accidents took us on a 6-million-year journey from something similar to a great ape to us, Homo sapiens.