AS William Cowan points out (Oxford Mail, June 22) there have been a number of evolutionary changes in the human line since we left our – and the chimpanzees’ – common ancestor some six million years ago.

These changes have been fairly well documented in the fossil record.

On the other hand, there are a number of similarities, such as the tailless torso, the rigid backbone, the opposable thumb, the teeth and bifocal colour vision.

The changes that have occurred in our line are, perhaps, no more or less than those that occurred in other species such as the differences in the horse and the rhino since they left their common ancestor – or the changes in the hippo and the whale since they parted.

Changes occur because a species is under threat, and it is thought that the human line has, more than once, faced extinction – borne out by the fact that we have a less complex set of genes than the chimp.

Our last major change occurred about 150,000 years ago, with an increase in brain size which has not only allowed us to survive but also to move into parts of the planet where, as a tropical animal, we would not be expected to go.

In the process we have done much damage, such as the burning of fossil fuels and the destruction of forests.

Will our larger brain help us to reverse this trend before our actions once again threaten our species?

Some people think that we are a superior animal but, since all life started as a single-celled organism nearly four billion years ago, all living things have proved that they are superior.

All life is different but equal.

Derrick Holt Fortnam Close Headington Oxford