90% of modern species, including humans, evolved within the last 100-200 thousand years.
Almost all species on Earth today came into being at about the same time, scientific study declares
Scientists are surprised to discover this,and I'm not sure what to make of it.
They looked exclusively at mtDNA, and only a fraction of the mitochondrial genome, which will be sensitive to lineage collapse. Although population genetics isn't my strong suit, I do think it is entirely possible for a largish continuous population to produce these observations without a bottleneck or expansion at any point.
A bit of background. The method they are using is called time to coalescence. They use the mitochondrial DNA mutation rate, generation time, and sequence variation to calculate how long ago these different mtDNA sequences shared a common ancestor.
The important bit to keep in mind is that mtDNA is only passed on through maternal lineages (from mother to daughter to granddaughter). If a female organism only has male offspring then that mtDNA lineage goes extinct. There is also the inevitable situation where organisms mate with their distant cousins which causes some mtDNA to become more dominant. Hence, you have this situation:
in case the picture doesn't appear:
Mitochondrial Eve - Wikipedia
Overtime, one lineage comes to dominate even in the absence of a population bottleneck. I think this is what is happening with their data. After speciation there is a lineage that comes to dominate the modern population through chance, and perhaps selection in some instances. This is exactly what the authors of the study offer as their explanation:
"Either of two evolutionary mechanisms might account for the facts: a) species-specific selection, or b) demographic processes acting independently of phenotype. "
https://phe.rockefeller.edu/news/wp-content/uploads/2018/05/Stoeckle-Thaler-Final-reduced.pdf
In science speak (from the article above):
"Several convergent lines of evidence show that mitochondrial diversity in modern humans follows from sequence uniformity followed by the accumulation of largely neutral diversity during a population expansion that began approximately 100,000 years ago. A straightforward hypothesis is that the extant populations of almost all animal species have arrived at a similar result consequent to a similar process of expansion from mitochondrial uniformity within the last one to several hundred thousand years."
Like the picture above, you start with a lot of variation and then lose that variation as mtDNA lineages die off.
