Early humans: Population size based on genetics

(mom) #1

I’m a non-scientist and in my reading of Genesis, I see how it’s possible God may have created a group of humans (through evolution or whatever) at the time of Adam’s creation (ie the implication that there are other people around- Cain’s wife, his building of a city, the people who might kill him if they find him). I’m not clear though about how genetics shows that the initial population was definitely more than 2 (Adam+Eve).

In this article from a few year ago, Doug Wilson asks:

So genetic evidence shows that humans descended from a group of several thousand about 150,000 years ago? Now when walking upstream like this, one wonders why they stopped right where they decided to stop.

This is because we could also say that genetic evidence shows that humans descended from about a billion people 200 years ago. And when we greet the several thousand ancestors from 150,000 years ago one wonders (does one not?) whether they had parents, whether they had common ancestors. What possible reason could we have for tracing our human ancestry to its point of origin, but then stopping a few centuries short? I’ll bet with a little scholarly work on this question we could go upstream a little bit further.

I can’t pinpoint quite what is wrong in his thinking, other than the “several thousand ancestors” being a bottleneck. But what does happen if we go back further, is there eventually a smaller population, albeit non-human?

(Christy Hemphill) #2

Welcome to the forum. Good question.

As I understand it, if you go back further in time to a smaller population, it cannot account for the genetic diversity that can be documented in the human population of today.

@DennisVenema @glipsnort or any others who know genetics, what links or articles would address this well?

(Dennis Venema) #3

As you go farther back, the population gets larger, actually. As we get back towards the common ancestral populations we share with chimps and gorillas, for example, we see a minimum population size of about 50,000.

Wilson is probably working with the common misunderstanding that eventually all species go back to an ancestral pair that “started” the species. Not so. That’s like thinking that new languages get started when two people start a new language. It doesn’t work that way.

(Chris Falter) #4

@BruisedReed - Dennis has actually provided a lot of detailed analysis about this question over the past few years. You should check out these articles on biologos.org:

(Christy Hemphill) #5

And here is the article that goes into lots of details with the “first speaker” of a language analogy, if you are interested.

(Stephen Matheson) #6

The one thing I would add to the excellent comments so far is that Wilson is missing something important about the calculation. It is about estimating the initial population, of a specific species (humans in this case, but these population genetic analyses are done in all sorts of contexts). Yes of course there are various populations of various kinds at various times in the past, that are our ancestors, but they aren’t human. And yes of course we are all descended from the humans who lived in 1066 AD, but that wasn’t the question. What the effective population analysis shows is that the initial population of humans, from whom we have inherited the genetic diversity of today, was of roughly a certain size with a certain level of genetic diversity.

(Jay Johnson) #7

Along the lines of population sizes, becoming human, and language evolution, this article had some interesting insights. I would love to hear your and @DennisVenema’s thoughts on it. The relevant bits:

"Resequencing studies have estimated the ancestral effective population size at 12,800 to 14,400, with a 5- to 10-fold bottleneck beginning approximately 65,000 to 50,000 y ago (although see ref. 15 for a bottleneck to only 450 individuals). It is generally assumed that the bottleneck occurred as a small group(s) with an effective population size of only approximately 1,000 to 2,500 individuals moved from the African continent into the Near East. … The loss of genetic variation during the Great Expansion is assumed to have resulted from the way the world was settled by hunter-gatherer groups that, after colonizing a new habitat and expanding there, shed small groups that founded new colonies nearby. This genetic sampling process led to the successive reduction of variation in the newly founded colonies, with the reduction being proportional to the number of founders. A set of 52 populations from all continents [the Human Genetic Diversity Panel (20) has been studied with two large sets of markers: 784 microsatellites (21, 22) and 650,000 SNPs (23)]. In a single reasonably homogeneous population, genetic diversity of biparentally transmitted DNA can be assessed efficiently by counting heterozygotes for all variants. The pattern of average heterozygosities of today’s populations suggest that, during the Great Expansion, there was a continuous decrease of genetic diversity with geographic distance from the place of origin in Africa (this takes account of the likely path of migration over land). The linear correlation between loss of genetic diversity and geographic distance from the origin of expansion in Africa is close to 90%. The Great Expansion is thus consistent with serial colonization and concomitant loss of genetic heterozygosity, a process called a serial founder effect (Fig. 2) (21, 24, 25)…

"Concordance Between Genes and Language.
The evolution of languages is rapid—in a few hundred years, a language may change enough to destroy mutual understanding between neighboring populations, or even between ancestors and their descendants 1,000 y later. Families of languages that are similar enough that most linguists recognize them as such have a common origin in the range of 10,000 y ago. There is a remarkable similarity between the linguistic tree and the genetic tree, confirming Darwin’s speculation that, if we knew the biological tree of humans, we could predict that of their languages (62). The first attempt to connect the two trees was made at a time when the linguistic tree was incomplete (63), but the similarity was clear and later shown to be statistically real (64). As might be expected, the geographical distribution of language families and that of genetic groupings of indigenous populations are also reasonably closely related.

“Beyond congruence in phylogenetic topology, other aspects of genes and languages show correlations stemming from deep population processes. A recent analysis of phonemic diversity in 504 worldwide languages shows that this diversity exhibits the same serial founder effect discussed earlier for genetic variation, namely a loss of phonemic diversity proportional to distance from Africa (65). Moreover, within Africa, the greatest diversity is in the southern central region. Although the regression of phonemic diversity on distance from Africa is not as strong as seen with DNA polymorphisms, this finding nevertheless suggests that the genetic and linguistic expansion from Africa could have been part of the same process…”

(Jay Johnson) #8

I suppose I should be more specific, so:

For @Christy
As a linguist, does it ring true to you that there is a loss of phonemic diversity proportional to distance from Africa?

For @DennisVenema
Does the population bottleneck of 1,000 to 2,500 individuals moving from Africa into the Near East about 65-50 kya fit your understanding of the evidence?
Does the decrease of genetic diversity with geographic distance from Africa make sense with your understanding?

Other opinions wanted and expected, of course.

(Christy Hemphill) #9

Not really. Languages have changed so much since the hypothetical proto-languages of 30,000-50,000 years ago. I have always heard that large phonemic inventories are more a function of languages being non-tonal or having relatively few vowels. It is more likely that languages lose tone over time than evolve tone. Africa, Asia, and the Americas are where tone languages are most common.

I found this article on evolutionary linguistics and Africa, if you are interested in skimming it. The author is pretty keen on polygenetic emergence of languages in Africa (hence, many unrelated languages attested in the migrating populations out of Africa).

(George Brooks) #10

A Greatttttttt article !!!

“In short, we can tell if everyone came from just two people at any time in the last 200,000 years. So did we? No.”

“This third independent method tells us that everyone alive today is related, but not to a single pair of people. We are related to a population that consisted of several thousand people with their several thousand combinations of these million genetic differences.”

“Here’s the real point of this. When you have one way of doing a calculation and you get a certain answer, perhaps you are justified in being a little skeptical. Perhaps you made a mathematical mistake, or maybe you made a faulty assumption. However, when you do your calculation using two totally different approaches, using methods with completely different assumptions, and each method gives you the same answer, you become convinced it is correct. Three, of course is just icing on the cake.”

“So that’s the situation we are in with regard to the human population size in ancient history. There was a bottleneck. There were likely fewer people alive during that time than the number of fans attending a typical NHL hockey game. (We don’t know if they were all together in one village, of course, but the total was small.) However, it was not two people. Our species diverged as a population. The data are absolutely clear about that.”

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(Jay Johnson) #11

Thank you for the reply and especially the link. I was dubious about the claim regarding loss of phonemic diversity, too. I’d already jotted down a few problems with it, but Mufwene covered those and a whole lot more. What she says actually fits pretty well with the scenario that I was considering, so it made my job easier, not harder. Thanks!

(system) #12

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