I have read that there are anywhere from 2 million to 100 million nonmicrobial species on this planet, with one supposed best estimate of 8.7 million. I have also read that an estimated 99.9% of species that ever lived are extinct. Surely we don’t have fossil remains of eight billion or so extinct species. So how do we know that such a high percentage of species that ever lived have gone extinct?
We don’t know-- That’s why they call it an estimate.
As to how they come up with that estimate, it’s probably mostly a matter of assuming that the number of species is roughly constant with time, and measuring the rate of evolution of the species we have now to estimate how long ago their ancestors would have been considered a different species.
Most extinct species are likely to be evolutionary branches with no living descendants.
Also, the number of species isn’t constant. There have been notable flowerings of diversity, and mass extinctions.
Anyway, talking about species over time is always going to be a somewhat semantic question. Like asking how many generations back was your ancestor who wasn’t in your family.
The original estimate appears to have come from David Raup. In his book Extinction: Bad Genes or Bad Luck?, he says the following:
(emphasis in original)
So the question then becomes, where did the estimate of 5-50 billion species that have ever existed come from. That estimate appears to come from a combination of the ratio of extant, fossilizable species relative to all extant species (itself an estimate), combined with the average species “lifespan” (cited anywhere from 1 - 10 million years) and estimates of average biodiversity over time. Obviously, these are all very fuzzy numbers, since none of the required values can ever actually be known.
I’d say that’s pretty much the definition of one.
Most extinct species are likely to be evolutionary branches with no living descendants.
Not at all. An extinct species has no living members but it can have descendants in newer species that are living. Homo erectus, for example is an extinct species, but is thought (at least by some) to be an ancestor species of Homo sapiens, us. Similarly some type(s) of extinct dinosaur species are ancestor species of modern birds.
I think Musicat meant that the number of species which do NOT evolve into other species (or a bunch of other species), but rather die out, is so proportionally overwhelming, that we might as well (statistically speaking) consider it part of the"definition" of a species, even though you’re right that it really isn’t.
Uh…no species evolves into another species “rather than” dying out.
Species is a murky concept, but when what we agree is a species is gone, it can have descendants or not, but it’s still gone. And many species that have descendants or species split-off from them are still extant at the same time as their ancestor/originator species. It’s not at all necessary for a species to die out and be replaced by descendants; in fact, probably most of the time there’s a period of overlap where Original Species and New Species both exist.
For example speciation often happens when a subset of a given species becomes isolated in a somewhat different environment. There’s no reason that the original population should be wiped out at the same time. For that matter, there’s no particular reason to suppose that the newer species will be the more successful one long-term.
Zoology (can I use that term in this case?) is ridiculously complex - so much so it makes an astronomer’s head spin.
Most of what they think they know really is an estimate built off a time line model looking for consistencies and patterns.
I find the misconception people have is that ‘species’ (for example), means a single type of animal and we of course then imagine it in it’s modern form. There may actually be a lot of descendants which we do not associate with it, because that ‘species’ already died out.