Human Chromosome Fusion Revisited

A reader has emailed me some questions about human evolution, in particular about the fusion of two ape chromosomes to form the human chromosome two. The email seems to be a follow up to this post of Razib’s. So, Let’s start with some general stuff and then work our way towards some specifics.


But first, let’s pose the questions:

“How long have humans been on this earth?” Or how long ago did we separate from the apes? (The short answer is we are not humans yet as we kill more of our kind than just about any other animal.) So I have narrowed my question down to this: When did the two primate chromosomes fuse to make the human/almosthuman #2 chromosome? Just within a few million years will be close enough for me. I was wondering if it would be 25 MYA or more like 6 MYA?

And:

What seems odd to me is that the combining of the 2a and 2b primate chromosomes to form the Homo #2 chromosome almost had to happen in more than one individual to raise a population. Seems like a difficult occurrence to happen more than once, and probably many times.

Obviously there are two types of data that bear on the question, paleoanthropological and genetic. Paleoanthropologically speaking, the question “how long have humans been on this earth” is a completely separate question from “how long have we been separate from the apes”. In paleoanthropological terms, we recent humans are called anatomically modern humans (AMH). Current evidence indicates that AMH arose some 100, 000 – 200,000 years ago. Perhaps the earliest fossil representatives of MAH occur in the Middle Awash region of Ethiopia. Specifically, two adult and one immature hominin crania were found dating to about 150,000 – 160,000 years BP. Additionally, a nearly complete crania has been found at Aduma (dating to 105,000 – 75,000 years BP), Gona(attributed to Middle Pleistocene – but the dates aren’t in yet) and Omo (cranial and postcranial fragments) dated to about 130,000 years BP. While we are on the subject of the middle Awash I should mention that in addition to all the Australopith material there is also Homo erectus represented by the 1 MYA Daka material and archaic hominins at Bodo dated to 600,000 years BP. So the question is, where do we draw the line and say this is “human” and this isn’t? Do we draw it at Herto? Daka? Bodo? or the Australopiths? Or do we go even further back to 6-7 mya and say Toumai is “human”? A paleaoanthropologist would answer that anything that occurs after the common ancestor of chimps and humans is part of the human lineage. Each of the above mentioned fossils have traits that link them to us. Take brain size. It is a truism in paleoanthropology that the australopiths show signs of increasing brain size. Creationist like to point out that the australopiths brains fall within the range of chimps and gorillas and ignore the fact that australopith brains that were approximately three times larger than those of Miocene apes. So really, the answer to the question of how long humans have been on this earth depends on how you define humans and what traits you use…
So what about the 2nd chromosome? Those of you who are unfamiliar with the issue can brush up here. Basically, what happened is that two ape fused, via Robertsonian translocation, into one human chromosome (the second). The email I received raised two questions about this. First, when did this happen? I have only read, perhaps, five papers on the subject. Only one of them tries to put a date on it (and that was basically an incidental mention) of between 6 MYA and 1 MYA. Overall the papers I have read have been more concerned with demonstrating that it did indeed happen. Second, can it happen more than once? The answer to that is yes. Robertsonian translocations, for example, are quite frequent and not always lethal. They come in two basic types – balanced (which are not detrimental) and unbalanced (which can be detrimental). You can find out more on Robertsonian translocations here. Additionally, as Robert Williams points out:

We have all heard that the horse and the donkey produce an infertile mule in crossing because of a different number of chromosomes in the two species. Well, apparently there is more to the story than we are usually told, because variations in chromosome number are known to occur in many different animal species, and although they sometimes seem to lead to reduced fertility, this is often not the case. Refs 5, 6, and 7 document both the existence of such chromosomal number differences and the fact that differences do not always result in reduced fertility. I can provide many more similar references if required. The last remaining species of wild horse, Przewalski’s (sha-val-skis) Wild Horse has 66 chromosomes while the domesticated horse has 64 chromosomes. Despite this difference in chromosome number, Przewalski’s Wild Horse and the domesticated horse can be crossed and do produce fertile offspring

So, to sum up, the event happened between 6 MYA and 1 MYA (although this paper narrows it to 6 -2 MYA). Nor does the translocation automatically imply reproductive isolation between those who have it an those who don’t…
Hope that answers your question…

4 Responses

  1. I think people put too much significance into the fusion event. I think it was neutral (or nearly neutral), given that it should have no effects on meiotic segregation. Also, the 2nd chromosome that all humans share is probably the result of a single fusion event. I know of no rearrangements that are homoplasies within a single lineage. Someone tried to argue that an inversion in Anopholes occurred twice, but that turned out to be bogus.

  2. Yes, that is what I was trying to get at. It’s neat that it could be detected, it’s one more piece of evidence for common descent, but by itself it doesn’t really say much …

  3. Hey Afarensis, thanks for the great post. And, yes, this helps answer the question as to how long we have been here.
    RPM, and your thinking is that the combining of 2a and 2b into Homo 2 may be similar to the parent of a translocation Down child? The parent with the translocation will have a normal phenotype but short one chromosome? Thanks for your comment too.

  4. As I understand it the translocation that causes Down’s Syndrome are unbalanced translocations – which are generally detrimental (see the link above)… Whearas the fusion event that created the second chromosome in humans was balanced. Chromosomes are tricky – for example, see the links at the bottom of this post on chromosome variation in one species of Israeli naked mole rat or this post on chromosome rearrangements and variation in diploid numbers in lemurs.

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