The paper is
here
Whilst it's been misreported, the findings are still quite intriguing and appear to change a general understanding - or at least mine. As I read it, 9 out of 10 existing animal species arose in the last 200,000 years. (Note, not at the same time 200,000 years ago.)
That's a better summary that the one in OP, but it's not quite right as I read it.
What I'd like explained among other things is how they get the 100,000 years by looking at DNA. Also what a DNA barcode is..
A DNA barcode is just a short stretch of DNA from one specific mitochondrial gene. Why is it called a barcode? This is a gene that's shared by all animals; and varies enough that it can be used to distinguish different species from one another. For this reason it's long been used in species identification, and scientists have gathered sequence data for about 5 million individual animals.
Because we have so many samples to work with, the authors thought this bit of DNA would be useful to study variation in a comparative way across species. They've previously tried to argue that the level of variation in the barcode region is a good proxy for genetic variation as a whole - using examples where we have the data to compare like humans and other other primates. While they accept that this is not firmly established, they take the attitude that this is the best measure we have until someone has sequenced millions of genomes of us to look in more detail.
So, we know that humans have lower than average genetic diversity for a big species. What Stoecke and Thaler discovered by comparing genetic diversity in this one region across many species is that human diversity is probably not as low as believed, comparitvely.
The average pairwise difference between humans in the DNA barcode region is about 0.1%. 90% of the species looked at have an average distance of 0.5% or less. In our closest relatives, chimps and bonobos, you're looking at about 1%. It's often assumed that humans are the outliers because of our unusual recent history; but if this work is correct it seems that it's actually the high diversity among chimps that's more unusual.
So what do they conclude from this? In simple terms, the unusual recent history of humans as a species may not be that unusual. Humans experienced a massive expansion in the recent geological past, in which they spread rapidly across the planet and the genes from one African population largely replaced that of other populations around the world.
This is where the number of years you were asking about comes from. It's nothing directly calculated from the DNA; and the authors don't put any number on it. All they're saying is that humans have a similar pattern of diversity to other species, implying a similarity in population history. Journalists are then looking up estimates for the origins of
Homo sapiens (50,000, 100,000, 200,000 years) and extrapolating that most species are as old as whichever estimate they picked.
But, important here is that we're not looking at exactly the same pattern of variation for all species, which is one of the main reasons the OP's 'simultaneous appearance' is silly. As mentioned, the range of average pairwise difference is between 0% and 0.5% for 90% of species. Humans are still towards the lower end of this range so are probably still 'younger' than most other species looked at.
I put 'younger' in inverted commas since the idea that what we're measuring here is the age of a species seems really stupid to me - not so much wrong as meaningless.
What are we actually looking at? We're looking at how long the mitochondrial genome has had to accumulate neutral variations - the more neutral variations, the longer it is since we can trace a populations mitochondrial diversity back to a small group of individuals. Is that the origin of a species?
Things that could cause reduced mitochondrial diversity would be population bottlenecks, rapid population expansions (like with humans) or selective sweeps.
This last one seems to me particularly important. Mitochondrial DNA is all inherited as a unit; so if any positively selected variant arises and spreads quickly through a population, it by necessity eliminates all neutral variation. You can't inherit the gene being selected for without also inheriting the other genes it happens to share a mitochondrion with.
Is that the origin of a species? If we have an existing population, and a new mutation appears in mitochondrial DNA which rapidly spreads in the population, did it just become a new species? That strikes me as an absurd thing to say. And the same thing applies for demographic models, for that matter. Cheetahs are going through a population bottleneck at the moment. Assuming the species survives into the future, there should be clear evidence there of reduced diversity at this point in the past; but would that mean that the cheetah species traces its origin to the 20th century? Were cheetahs in the medieval era a different species then?
This is why I think it makes no sense to say anything about origins of species in connection with this work. What they have actually demonstrated is that animal species seem to regularly (on geological timescales) go through phases of greatly reduced genetic diversity; which means either population bottlenecks, rapid expansions or selective sweeps. And it should be made clear that we're only talking about mitochondrial DNA here - it's not established that the same pattern will hold in the nuclear genome. This means the whole pattern could be explained simply by adaptive selection of mtDNA being more common than is usually assumed (a point argued by some geneticists already).
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ABE: To be clear, I'm not calling the paper's claims absurd, but the way this has sometimes been reported in the press. The actual conclusion in the paper is worded much more sensibly:
quote:
Similar neutral variation of humans and other animals implies that the extant
populations of most animal species have, like modern humans, recently passed
through mitochondrial uniformity.
This seems to be the actual quote to the press from one of the authors which has been twisted into what you see in the OP (via
phys.org):
quote:
Our work suggests that most species of animals alive today are like humans, descendants of ancestors who emerged from small populations possibly with near-extinction events within the last few hundred thousand years."
Edited by caffeine, : No reason given.
Edited by Adminnemooseus, : Fix link.
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