What do scientists mean when the say 99% of all species went extinct?

Well Adrian this is obviosuly an estimate really, but probably a very accurate one.

In simple terms we have had life on Earth for about 4.5 billion years. Although for a long time life would have been simple in structure and with few species, that is a very long history. Looking at the fossil record we can tell that most species don't actually last very long from the time they first appear till they vanish from the rocks (for example the average is about 4 million years for mammals).

Now diversity clearly varies over time (during big extinctions there won't be many species left for quite a while), and currently there are estimated to be somewhere between 30-50 million species on Earth. Now given that each species can only be expected to survive from around 0.5-5 million years, its perhaps not surprising that taken over 4.5 *billion* years, over 99% of all species that have ever lived are no longer with us.

I believe the figure is actually 99.9999....%!

I realise this seems like a paradox in some ways as the history of life in a general sense has been about ever increasing numbers of species. It might be better to think about this in terms of building a family tree. As you keep on adding extra generations back in time then the number of living members gets increasingly less and less. Of course our great-great grandparents didn't go extinct as such, but the analogy holds for species in that some of them were ancestors of living species, whereas others never had children and so have no living descendants.

So in truth, when we say that 99.9999% of species that have ever existed went extinct we really mean two things. Firstly, most 'types' of organism (animal, plant, fungi, whatever) only last a few million years at most before their way of life becomes outmoded as environments shift or competitors appear. However, 'extinction' can also mean evolution into a totally new form. As the old form (species) disappears this is still an extinction as a particular way of life is lost, although there is still a carry over of genetic material.

Zooming out to look at the bigger picture this means that everything living today is the result of an unbroken sequence of ancestors that goes back at least two billion years. You, me and everyone else on this forum are part of a lucky chain of survivors going back countless generations. Many others didn't make it (even very large groups that are abundant in the fossil record such as the dinosaurs, trilobites, ammonites, graptolites etc.) and most scientists agree this is more to do with luck than some inherent superiority of modern forms.

Last edited by Graeme Lloyd (21st Jul 2007 16:26:30)