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In the opening chapter of Sapiens, Harari writes:

Just 6 million years ago, a single female ape had two daughters. One became the ancestor of all chimpanzees, the other is our own grandmother.

Is there evidence that this is literally true? There seem to be two ways in which it could fail:

(a) Weak version---there is an individual last universal common ancestor, but it's not an ape: that is, all humans (and chimps) share a single individual creature as an ancestor, but that creature was not an ape. It was some earlier ancestor whose descendants would go on to be apes.

(b) Strong version---there is no individual last universal common ancestor: if tracing the entire history of life on Earth and assuming some "stopping point" where we saw life emerge as (e.g.) from a volcanic vent primordial soup, we would find distinct creatures emerging simultaneously in a population, with similar but distinct genetic code, from one of whom some species (such as humans) evolved, and from another of whom others evolved.

Is there evidence that neither is the case? First, (1) that there is an individual creature, rather than a population of creatures, from whom all life descended. If that is true, then it follows that all humans have an individual as a common ancestor, since you can at worst follow the line of descent back to the primordial individual. And, next, (2) that the last common ancestor of all humans (and chimps) was also an ape, rather than say some other animal.

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Just 6 million years ago, a single female ape had two daughters. One became the ancestor of all chimpanzees, the other is our own grandmother.

Short answer

Whether or not the claim is true actually depends upon the specific DNA sequence you are considering. The claim is true for sequences without recombination (such as mtDNA and most of the Y chromosome). The claim does not hold true for most of the genome and it is therefore misleading.

Long answer

Basic knowledge required

First, if you don't know the concepts of

  • allele
  • haplotype
  • ploidy
  • selection
  • genetic drift
  • segregation
  • recombination and crossover

, then you might want to have a look at a short introduction of evolutionary biology such as Evo101 for example. The below answer will make more sense to you with some a priori knowledge.

Coalescence

Imagine there is today a population of $N$ individuals. We are going to look at this population backward in time (which is a little bit unintuitive). For simplicity, we will assume asexual reproduction, no recombination, absence of selection (only drift) and constant population size of $N$ individuals. The assumption of no recombination is key here. The other assumptions are details just to make my job at explaining coalescent theory easier.

We start at time t=0 and are going backward in time. If two individuals are siblings at $t=0$, then, it means that in the previous generation, they share the same parent. Therefore, if we connect individual from generation $t=0$ to their respective parents at generation $t=-1$, we see that some group of individuals (the siblings) "coalesce" to fewer individuals (their parents). Individuals at generation $t=-1$ that are not connected to individuals at generation $t=0$ are those who did not reproduce (or who did not leave offspring that have survived until adulthood).

You can keep iterating over generation $-2$, $-3$, $-4$, etc... At each generation, the number of individuals that are connected (that our ancestors) to present day individuals is diminishing as more and more coalescence happen. After a sufficient number of generations, eventually, only one parent will remain. This single individual will be the ancestor of all present individuals.

If you enjoy math, then note that such process is actually a simple a branching process for which it is relatively easy to calculate the expected time (and the whole PDF) to the next coalescent event and the expected time (and the whole PDF) until all individuals in the population coalesce.

So, assuming no recombination, if you consider individuals in a modern population, then you can be sure that all individuals share a common ancestor who is a single individual, at some point in the past.

Incomplete lineage sorting

The process of all individuals coalescing into a single ancestor takes a little bit of time. For recently diverged species (say species A and species B), it is possible that this common ancestor is older than the time of divergence between species. If this is the case, it is then incorrect to think of a mother who gave birth to two individuals, one becoming the ancestor of all individuals of species A and one becoming the ancestor of all individuals of species B. In such a case, we talk about incomplete lineage sorting.

Note that when we talk about 'incomplete lineage sorting', we are generally talking about specific genes as not all genes coalesce the same way when there is recombination and segregation (see below)

Recombination

Now the trick is that in the presence of recombination, different sequences coalesce differentially. It also means that when you look at two sister species, some sequences can show incomplete lineage sorting while others don't.

The presence of recombination therefore makes the claim false!

Do we recombine?

Yes, all of our genome recombines to the exception of two sequences

  1. mitochondrial DNA (mtDNA)
  2. Y chromosome not including the Pseudo-Autosomal Region (PAR)

mtDNA is maternally inherited (without crossover) and transmitted to all offsprings, while the Y chromosome is paternally inherited (without crossover) and transmitted to sons only.

So, is the claim true?

mtDNA

It is true that there was an ape who had two daughters. One daughter's mtDNA became the ancestor of all mtDNA we have in humans today, the other daughter's mtDNA became the ancestor of all mtDNA that exist in chimps populations today.

Of course, this daughter is the common ancestor of all modern mtDNA but is not necessarily the Most Recent Common Ancestor (MRCA). In humans, the MRCA to all modern mtDNA is called Mitochondrial Eve (mt-Eve or mt-MRCA).

Y Chromosome

The same claim is true for the Y chromosome (although I am not sure there was really not a single recombination event happening on the Y chromosome since). The MRCA to today's all Y chromosome is sometimes called Y-chromosomal Adam (Y-MRCA).

The names 'mt-Eve' and 'Y-chromosomal Adam' have of course been chosen in reference to the creation myth of Abrahamic religions.

Rest of the genome

The Y chromosome and mtDNA together represent less than 0.2% of our genome. The rest of our genome does recombine.

The claim does not necessarily hold true for 99.8% of the genome. I would therefore tend to think that the claim is misleading at best, wrong at worst.

Adam and Eve

We can call Adam the MRCA of all Y chromosomes today. Only Y chromosomes, not the rest of the genome. We can call Eve the MRCA of all mtDNA today. Only mtDNA, not the rest of the genome.

Of course, Adam and Eve did not live in the same time period and did not live at the same place. Adam and Eve never met. And of course, there were plenty of other humans around. They are in no way the first two humans.

Other popular source of knowledge

PBS Eons - The Two People We're All Related To

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  • $\begingroup$ This is interesting. I will read more. I am struggling intuitively to understand the distinction between descent of particular DNA components versus of whole individuals. Shouldn't the question be answerable just by looking at a "tree" of individuals, without having to consider DNA? We might need godlike powers of historical observation, but if we had them there would be an answer at the individual level. So the DNA story may substitute for the lack of godlike history knowledge, but should it not ultimately tie back to individuals? (1/2) $\endgroup$ – Philip Jan 22 '18 at 16:41
  • $\begingroup$ (2/2) I.e., not who is the ancestor of which Y chromosomes, but just which individual(s) is/are the ancestor of which individuals. Separately, I'm confused about the coalescence argument. Consider times t = 0 ... N, and two individuals A_0 and B_0 at the "dawn of life," and each individual X_t asexually produces a child who will live at X_{t+1} and then dies. Then today I could have two individuals A_N and B_N who do not share a common individual ancestor. I believe you that this counterexample fails, but I don't clearly see what evidence we know that establishes its failure. $\endgroup$ – Philip Jan 22 '18 at 16:41
  • $\begingroup$ @Philip remember sex is involved with most individuals, you have two parents, 4 grandparents, 8 great grandparents, ect. As you go back eventually every individual in the population that reproduced is your ancestor. the ones that did not reproduce didn't pass on their genes. For a lot of asexual organisms "individual" as a concept kinda breaks down, a bacteria splits in half forming two, which daughter cell is or is not the same individual as the parent? $\endgroup$ – John Oct 30 '18 at 1:32
  • $\begingroup$ there might be a woman who first had the mutation that created the gene common to all mitochondria in humans, but she had to have sex with someone, and so do her children and both of them has to have parents, and grand parents, and so on and so forth. As you go back it becomes increasingly more likely with each generation that they will have an ancestor in common, If you go back far enough they basically have to have common ancestors because the population size is finite but the but the number of ancestors increases exponentially. most importantly. $\endgroup$ – John Oct 30 '18 at 1:51
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It is easy to describe a scenario in which there is no single individual who can be called "the" ancestor of all individuals in a much later generation. Let's say there was, indeed, one individual female who had a mutant form of mDNA that occurs in all of us now; and also let's say that that particular mutation never occurred again (a BIG assumption!). Indeed that female would be an ancestor to all of us; but certainly not the only ancestor. That one female's daughters and sons each carried only half of her genetic information; the other half came from their father(s); and each of those fathers had their own mothers. Each of us is a genetic mosaic of a very large number of individuals.

Even if all humanity descended from one tribe of early hominids, with no cross-breeding between tribe members and outside tribes (very unlikely), we would all very probably carry genes contributed by almost all of the members of that tribe.

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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

  • $\begingroup$ "Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted." Please be more specific. Which assertions require references? This is very basic genetics. $\endgroup$ – S. McGrew Oct 31 '18 at 14:20

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