Penny Gray, of Rangi Ruru School, asks :-

For how many years have people been on Earth?

Judith Littleton, a biological anthropologist at the University of Auckland, responded.

You would think that the one thing scientists would definitely know is how long people have been on this earth but nothing is that simple. Modem humans are known as Homo sapiens (our species name). The oldest fossil that we know belongs to this species comes from Ethiopia (its called Omo Kibish 1) and is dated to around 130,000 years ago.

But modern Homo sapiens is only one member of a very large family tree which started somewhere between 5 to 7 million years ago when the ancestors of the modern gorillas, chimpanzees, and humans moved into different environments.

We don't know what this common ancestor looked like. It was probably a fairly small animal, one metre tall maybe, with a small brain, that climbed around trees on all fours but could also stand upright for short periods of time.

Why the family line that finally led to modern humans became separate is unclear but over the remaining five million years different species developed and became extinct as the environment changed. So our family tree is a very bushy one.