About Rationally Speaking

Rationally Speaking is a blog maintained by Prof. Massimo Pigliucci, a philosopher at the City University of New York. The blog reflects the Enlightenment figure Marquis de Condorcet's idea of what a public intellectual (yes, we know, that's such a bad word) ought to be: someone who devotes himself to "the tracking down of prejudices in the hiding places where priests, the schools, the government, and all long-established institutions had gathered and protected them." You're welcome. Please notice that the contents of this blog can be reprinted under the standard Creative Commons license.

Saturday, July 22, 2006

Homo-chimp hybrids?

A group of researchers at Harvard and MIT, headed by Nick Patterson (Nature 29 June 2006, pp. 1103-1108) has uncovered evidence of ancient hybridization between humans and chimpanzees. Although, predictably, the paper has raised unspecified (and rather silly) “ethical concerns,” it is actually one potentially important milestone in our quest to find out where we really came from (as opposed to those myths involving gods spitting on mud and such).

Patterson and colleagues were able to use the latest data from both the human and chimp genome projects to reconstruct the evolution of many genes across all chromosomes present in the two species. They found that the data are consistent with a more recent split from our common ancestor than indicated by the available fossil record, probably less than 6.3 million years (assuming you believe that the earth is more than 10,000 years old, of course).

Even more strikingly, the Harvard/MIT team found that the X chromosome (the one involved in sexual determination) diverged more recently than the estimated time of split of the two species. How is this possible? The most likely interpretation is that humans and chimps kept exchanging genes (i.e., hybridizing) after the separation of the two lineages. When this happens in other species, one of the results is strong natural selection on the sexual chromosomes, yielding the sort of data observed by Patterson et al. The reason the sexual chromosome is under more intense selection, especially after hybridization, is that half of the population (in primates, the males) only have one copy of that chromosome, which means that any deleterious gene has immediate effects on fitness, and is therefore promptly selected against. This is the same reason why so many human genetic defects (from color blindness to hemophilia) are much more common in men than in women (the latter have a second X chromosome, which shields them if it carries a functioning copy of the same gene).

This is a great example of how science actually provides us tantalizing clues about fundamental questions, by clever detective work that yield strong circumstantial evidence, and sometimes even permit us to arrive at conclusions beyond reasonable doubt. The “other” way of proceeding, apparently more satisfying to a large number of Americans, is to listen to the latest tall tale by the local preacher, based on the personal (and uninformed) interpretation of a book written by ignorant people thousands of years ago. Go figure.


  1. This one will get the religious folk, especially the fundies tearing their hair out! How I love it!

  2. Just one more proof that "evilution" is wrong, wrong, wrong! :O)

    OK, back to normal now. As Massimo points out, it has happened all over the place. This would be no newsthy item at all had it been about, let's say, horses and zebras...

    This is only news because we cannot digest the fact that we are animals, and as such we followed the rules of nature for most of our existence. On second thoughts, we still do follow nature's rules, since we are part of nature after all and most people's delusion of grandeur notwithstanding.


  3. You're going to Hell for this one, Max... :D
    Really, though, isn't the term "hybrid" misleading, in the sense that (contrary to fundie beliefs) speciation is not a digital, 1-or-0, ON-or-OFF, human-or-animal (sic) scenario, but rather an anlogue semaless continuum, which joins modern humans to ape-like ancestors (including "hybrids") and the modern Great Apes? (And, further back, to other mammals, and reptiles, and all other concestors all the way back to the primordial ooze.)
    It is the Manechian worldview that is particularly pernicious, for (to tie to an earlier one of Max's posts this month), it leads people deludedly to label embryonic stem cells as "human", and it is from there that much evil flows.

  4. (the latter have a second X chromosome, which shields them if it carries a functioning copy of the same gene)

    My limited understanding of genetics tells me that one copy of the X chromosome in every female cell is deactivated (they become barr bodies, if I recall correctly). Are there any processes in the cell that would allow for the preferable crhomosome to remain activated?

    Just a curiosity.

  5. Cody, you are correct on the de-activation of the X. There is some evidence that this does not happen at random (as previously thought), but is controlled by epigenetic factors. I don't know much more about it.

  6. "humans and chimps kept exchanging genes (i.e., hybridizing) after the separation of the two lineages."

    Any port in a storm! ;-)

  7. I think our present federal administration (including a goodly number of Sens. and Reps.) are evidence that this hybridization continues at a rampant rate.

  8. The example I remember reading about was coloration in calico cats. Cells with a de-activated X chromosome of one color would have an activated X chromosome of the other color. Which chromosome that gets de-activated varies from cell to cell (or at least different clusters of cells), and this is what's said to give calicos their unique coloration.

    Could the differential de-activation of x chromosomes result in some kind of lessened impact for x-linked diseases? Partial color-blindness, perhaps?

    Where would you suggest looking for more information on this topic?

  9. Cody, a good starting point would be a graduate-level textbook on human genetics (any would do), look up where they talk about X chromosomes and track down some of the technical literature. Then let us know!

  10. On the X chromosome and Barr bodies, at some relatively early stage in embryonic development one or the other X chromosome becomes permanently deactivated in each cell. The descendants of each of those cells will have the same X chromosome deactivated, which accounts for the color pattern of the callico cat, as cody said.

    For any given patch of tissue, one or the other of the X chromosomes is inactive, but over the entire organism, both X chromosomes are available to produce their gene products. If you need some enzyme produced on the X chromosome and one copy is defective, you have another copy, albeit in another cell, producing that product. For most traits this doen't seem to be a problem.

    One amusing consequence is that some women can see more colors than other women, and all men. There are two slightly different alleles for one of the color detecting pigments in the eye. Since women have two copies, both alleles can function, and some colors can be differentiated by these women that can't bbe percieved by men or by women who have two coppies of the same allele.

  11. That's why we used to call the girls "chimeras" back in college. :-)

    Interesting this thing about the color, I had heard about it before, but forgotten. Recently I've read an article on Scientific American talking about color vision in vertebrates, and concentrating on birds. We poor primates have 3-color vision, but birds (and reptiles, amphibians and fish) have 4-color vision, so they see the world pretty differently from what we see. Must be cool. Is like the example mentioned, of women who can see more colors, but way more drastic. Them birds can see some UV...

    Regarding the inactivation of the X chromosome, recent research has shown that 10 to 15% of the inactivated X is actually expressed, it's actively producing... whatever it's producing. I don't remember if these are always the same bits of the chromosomes or not. So women have some genes being produced with 2x the dose present in men.

    "You and I, we are strangers by one chromosome", as Rush said, is actually even more than that...


  12. "The reason the sexual chromosome is under more intense selection, especially after hybridization, is that half of the population (in primates, the males) only have one copy of that chromosome, which means that any deleterious gene has immediate effects on fitness, and is therefore promptly selected against."

    Aren't you even interested in how nature arrives at just about a 50/50 ratio of males vs. females, and especially in the human population? I sure am. I know that the ratios tip a bit in the favor of females, (unless one is talking about an asian/ indian country where females are selected against out of fear and superstition) but it's absolutely ingenious that “female” just turns out to be the default setting when only females can carry offspring.

    It's almost as if someone deliberately designed the whole concept of gender and the need for intelligently spaced allotments of each gender!

    And, YES, of course I do believe that. If you don't, feel free to explain the ratio balance.

  13. Here follows some excerpts from Wikipedia articles on Games Theory(1) and Sex ratios(2).

    (1)"In biology, game theory has been used to understand many different phenomena. It was first used to explain the evolution (and stability) of the approximate 1:1 sex ratios. Ronald Fisher (1930) suggested that the 1:1 sex ratios are a result of evolutionary forces acting on individuals who could be seen as trying to maximize their number of grandchildren."

    (2)"Males and females are produced in approximately equal numbers in most species with separate sexes, regardless of the mechanism of sex determination.

    R.A. Fisher, in his 1930 book The Genetical Theory of Natural Selection outlined a model that explains the (approximately) 1:1 sex ratio commonly seen.[4] Fisher's argument goes as follows (cf. The Genetical Theory of Natural Selection, pp. 142-143): Parents incur some level of expenditure to raise offspring. The reproductive value of male offspring is equal to the reproductive value of female offspring, since each individual of the following generation has exactly one male and one female parent. It follows that parental expenditures for males and females are equal. For, if, say, the parental expenditure for males were less than the parental expenditure for females, then a parent with a congenital bias to generate male offspring would produce, for the same amount of parental expenditure, offspring with a higher reproductive value than a parent with a "female bias" would. Hence the sex ratio would be raised until parental expenditures for males and females become equal. It follows that, if parental expenditure for individual male and female offspring are roughly the same (as is the case, for example, in humans), then the sex ratio is approximately 1:1, or 1/2 by the definition commonly used in biology."

    This may or may not be the definitive answer to the question of sex ratios, but it should at least show how natural selection might be employed to explain such things.

  14. suf

    Tho your entire way of thinking this through is quite reasonable, workable or successful ratios in game theory are essentially, and nothing less than a product of intelligence.


  15. Cal

    It's just an equilibrium, isn't it? What's intelligent about equilibrium?

  16. Actually humans slightly overproduce sons at birth, roughly 105 boys for every 100 girls. But since boys have slightly higher mortality (possibly due to the unprotected X) the sex ratio is balanced at roughly 18 years of age.
    In species with XY systems it's not so hard to understand (from a proximate mechanistic point of view) at all why the sex ratio is close to 1:1, since typically half the sperm carry X and the other half Y, simply due to random Mendelian segregation during meiosis. If the two types of sperm are equally good at fertilizing eggs, you get the expected ratio.


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