Is it true.. species of any type, except viruses/bacteria have a lifespan of 2.5 million years on average?

Question

Not including natural disasters, meteors etc..does evolution simply reach an apex and for some unknown reason the speices dies out?

Answer 1

Based on my experience with fossils of planktonic single-celled marine organisms (which are incredibly abundant and well studied - they are used in petroleum exploration) I would say 2.5 m.y. is a gross under-estimate. Off the top of my head I would say 10-15 m.y. would be a more accurate average in my experience, but there is wide variance.

As to the statistical estimates from benthic marine invertebrates and the more common terrestrial vertebrates, the problem is that most of these species only lived in specific environments. Generally speaking in sedimentary geology, there are large scale, regional cycles of the environment over 2 to 5.m.y. (third-order sequences in petroleum exploration, linked to sea-level change). Although correlation doesn't necessarily imply causation, these cycles obviously caused huge local environmental change (and are associated with global climatic change), and at the same time, they insert a complex patchiness in to the fossil record of most organisms.

So personally I'm a bit sceptical of such averages (and I've seen other ones eg. 7/8 m.y., 12 m.y, 20 m.y.). But if they are true, then it seems most likely that they are linked to environmental change.

Answer 2

The problem is that the fossil record is, by its nature, fragmentary and does not perfectly preserve the full range of any species' history as an extant organism. For some groups, such as the incredibly diverse array of species of bacteria, or many of the insect groups, this problem is compounded by the incredible scarcity of these species of these fossils, or the inability to identify them to species if they are preserved.

On top of that is the potential for wide ranges in differences between groups. If the many species of bacteria last for 20-30 million years (not true, just given as an example), while species of another group last for only 10000 years, then the average species lifespan is not a useful measure. It's like taking the average income of a street where 50 McDonald's workers and Bill Gates live. The 'average' income might be $2,000,000 a year, but no one on the block actually makes that amount. In this case, the mode would probably be a better measurement, rather than the median.

Also, there are differences in what different groups call a 'species'. When many of the conservation groups call for protection of an endangered species like the Florida panther, what they are really refering to is a subspecies of a larger species, Felis concolor aka the cougar. Using subspecies and species interchangeably royally screws up any kind of 'average' measure like this, and really confuses the issue when they talk about things like '17,000 species may go extinct before the end of the century'. It almost makes such measures meaningless.

Even if species composition changes every 2.5 million years on average, it doesn't necessarily represent extinction events. In many cases, it is probably due to gradual evolution, with slow incremental environmental shifts being matched by slow, incremental evolution within various populations that results in sufficient variation from the original phenotype that the population is now considered a different species from what they were 2.5 million years ago. The splitting of these into different species is another result of the fragmentary nature of the fossil record, as well as (to some extent) the tendency of scientists to split species, rather than lump (a paper describing a new species is sexier than a paper that merely describes a variation of an existing species).

So basically, the number is a wild estimate, based on incredibly incomplete data, and the variation of lifespans between different phyla or kingdoms probably renders an average meaningless.

It's not quite a number pulled out of their bums, but it's close.

Answer 3

The average lifespans of species vary significantly across the animal kingdom. The average is 5 to 10 million years, but mammals usually last closer to 1 or 2 million years.