When and where did we begin? What is a Homo sapiens anyway?

July 17, 2018 8:33 AM

The New Story of Humanity's Origins in Africa comes from several new discoveries, which suggest that our species didn’t arise from a single point in space. Instead, the entire continent was our cradle. In reviewing and summarizing recent developments across multiple fields, 23 scientists pose the questions Did Our Species Evolve in Subdivided Populations across Africa, and Why Does It Matter? Researchers have determined that much of human DNA comes from Neanderthals, and at least two other hominid species. And with these re-evaluations come others, including the discovery of ancient (2.1 million year old) tools in China that suggests early hominims left Africa earlier than previously thought.
posted by filthy light thief (15 comments total)

OK so one upside to being alive in this time: our knowledge of our early human story is getting more and more interesting. Thanks for these links filthy light thief!
posted by Meatbomb at 9:38 AM on July 17, 2018 [1 favorite]

I never knew that the Africa origin thing meant to imply we all came from the same single population. I figured it was different folks from different groups all over anywhere near what we today call Africa. In conjunction with the other homos we fucked into ourselves, early human history is always fascinating to me. I hope some of these potential sites currently ignored get some time with the right paleontologists.
posted by GoblinHoney at 11:00 AM on July 17, 2018 [1 favorite]

Researchers have determined that much of human DNA comes from Neanderthals, and at least two other hominid species.

I would rephrase this to better reflect the data. Many people from certain regions have a small percentage of non-Homo sapiens DNA.

Prüfer and Pääbo say that, based on the new high-quality genome, modern populations carry between 1.8 to 2.6 percent Neanderthal DNA—that’s higher than the previous estimates of about 1.5 to 2.1 percent. More specifically, East Asians have about 2.3 to 2.6 percent Neanderthal DNA, while people from western Europe and Asia have retained about 1.8 to 2.4 percent DNA. African populations have virtually none because their ancestors did not mate with Neanderthals.

posted by hydropsyche at 12:29 PM on July 17, 2018 [8 favorites]

Thanks for the correction, hydropsyche! That was a bad pull-quote on my behalf.
posted by filthy light thief at 12:38 PM on July 17, 2018 [2 favorites]

about 2.3 to 2.6 percent Neanderthal DNA

So, uh, how does that square with the other well-known factoid that we have 95% of our DNA in common with chimpanzees?
posted by sfenders at 1:42 PM on July 17, 2018 [1 favorite]

I still haven't managed to get a hold of the actual study. This summary of the same study answers your question using a different way of discussing DNA from the previous article I quoted:

The results from the new studies confirm the Neanderthal's humanity, and show that their genomes and ours are more than 99.5 percent identical, differing by only about 3 million bases.

"This is a drop in the bucket if you consider that the human genome is 3 billion bases," said Edward Rubin of the Lawrence Berkeley National Laboratory, who led one of the research teams.

For comparison, the genomes of chimpanzees, our closest living relatives, differ from humans by about 30 million to 50 million base pairs.

posted by hydropsyche at 2:57 PM on July 17, 2018 [4 favorites]

That doesn't answer the question so much as highlight why it's puzzling. 3 million out of 3 billion base pairs would be ~99.9% identical DNA between Neanderthals and Homo sapiens, so by what measure is that 2% "Neanderthal DNA"? I guess I can imagine how they might find some amount which definitely did originate with Neanderthals, but how do they decide that the other 98% definitely didn't? And if chimpanzees have 50 million out of 3 billion base pairs not in common with us then that's >98%, and adding up those numbers would leave less than 0% of our DNA that didn't come from either Neanderthals or some common ancestor with chimps?
posted by sfenders at 3:22 PM on July 17, 2018

Numerator common base pairs in the 3mm that usually vary, denominator 3mm.
posted by MattD at 3:43 PM on July 17, 2018 [1 favorite]

My guess is that on average Homo sapiens and neandertalensis vary by ~3 million bases, so those are the bases we look at for % similarity between the species. Some folks, especially East and South Africans, are 100% different from Neanderthals in those 3 million. Other folks, especially Europeans and Asians, are more like 98% different from Neanderthals in those 3 million.

Neanderthals and Homo sapiens would have exact the same % (indeed, the exact same base pairs) in common with chimpanzees, all deriving from the same common ancestor when Hominins split from the other apes.

These two ways of saying it are also just difficult to compare because of rounding errors. ~3 million, ~50 million, ~3 billion--what exactly is the error on all of those? Enough to mean the percentages in the first article are a better approximation than your quick division.
posted by hydropsyche at 3:50 PM on July 17, 2018 [2 favorites]

Thanks, that'd explain it.

More than just rounding errors involved, perhaps: The first source I came across when attempting to decide whether my memory of how much DNA we're supposed to have in common with chimpanzees had any relation to reality says it's more like 95% rather than the oft-cited ~98%. The higher number apparently came from taking a convenient shortcut to estimating it, while newer techniques allow them to more or less count them all in what I suppose was a representative sample.
posted by sfenders at 4:14 PM on July 17, 2018 [2 favorites]

I have wonderful DNA thanks for asking.
posted by Homo neanderthalensis at 5:22 PM on July 17, 2018 [11 favorites]

Part of the problem discussing shared genes and inheritance is that there are multiple concepts of "sharing" genes. Genes can (and do) move around due to recombination and other events, and they remain functional, but they are not just a bag, there is structure that only changes slowly over time. To give an example, our "chromosome 2" looks like two chimpanzee chromosomes fused together.

So you can ask "how many genes do we share in common with chimps, regardless of how they're organized on chromosomes" and get a number around 95%.

But you can also look at the structure of the genes in great detail, and use that to track lineage (both descent, and to a certain extent time since intermixing). It's surprisingly precise, to the extent that it can track movement of ancient populations (as long as we can find human remains to sequence). Using this technique we can see that some modern human populations have a couple percent of their genome derived through direct decent from Neanderthals.

So: we share most of our genes with Neanderthals because we share common ancestors with them that we derived the same genes from, but we directly derive a couple percent from them.
posted by Humanzee at 6:23 PM on July 17, 2018 [10 favorites]

posted by Humanzee

This is somewhat beyond eponysterical
posted by saturday_morning at 4:10 AM on July 18, 2018 [4 favorites]

Terrifying-sterical?
posted by Homo neanderthalensis at 5:52 AM on July 18, 2018 [2 favorites]

If you've done 23 & Me it has a section for your Neanderthal ancestry that says:

You have 303 Neanderthal variants.
You have more Neanderthal variants than 87% of 23andMe customers.
However, your Neanderthal ancestry accounts for less than 4% of your overall DNA.

For some reason, the "However" makes me laugh.
posted by elsietheeel at 12:38 PM on July 18, 2018 [4 favorites]