How Archimedes' lever explains human evolution

By Razib Khan | November 5, 2011 6:38 pm

Last August I had a post up, The point mutation which made humanity, which suggested that it may be wrong to conceive of the difference between Neanderthals and the African humans which absorbed and replaced them ~35,000 years ago as a matter of extreme differences at specific genes. I was prompted to this line of thinking by Svante Pääbo‘s admission that he and his colleagues were searching for locations in the modern human genome which differed a great deal from Neanderthals as a way through which we might understand what makes us distinctively human. This sort of method has a long pedigree. Much of the past generation of chimpanzee genetics and now genomics has focused on finding the magic essence which differentiates us from our closest living relatives. Because of our perception of massive phenotypic differences between H. sapiens and Pan troglodytes the 95-99% sequence level identity is thought by some to be perplexing. Therefore models have emerged which appeal to gene regulation and expression, or perhaps other forms of variation such as copy number, to clear up how it can be that chimpanzees and humans differ so much. Setting aside that the perception of difference probably has some anthropocentric bias (i.e., would an alien think that chimpanzees and humans are actually surprisingly different in light of their phylogenetic similarities? I’m not so sure), it doesn’t seem to be unreasonable on the face of it to plumb the depths of the genomes of hominids so as to ascertain the source of their phenotypic differentiation.

But can this model work for differentiating different hominin lineages? Obviously there’s going to be a quantitative difference. The separation between chimpanzees and modern humans is on the order of 5 million years. The separation between Neanderthals and modern humans (or at least the African ancestors of modern humans ~50,000 years B.P.) is on the order of 500,000 years. An order of magnitude difference should make us reconsider, I think, the plausibility of fixed differences between two populations explaining phenotypic differences.

 

Backing up for a moment, why do we think there might be fixed differences between Neanderthals and modern humans? The argument, as outlined in books like The Dawn of Human Culture, is that H. sapiens sapiens is a very special lineage, whose protean cultural flexibility allowed it to sweep of the field of all other hominin sister lineages. The likelihood of some admixture from these “dead end” lineages aside, this rough model seems to stand the test of time. Consider that the Mousterian technology persisted for nearly 300,000 years, while the Oldowan persisted for 1 million! In contrast, our own species seems to switch and improve cultural styles much, much, faster. Behavioral modernity does point to a real phenomenon. The hypothesis of many scholars was that there was a genetic difference which allowed for modern humans to manifest language as we understand it in all its diversity and flexibility. The likelihood of this seems lower now that modern humans and Neanderthals have the same variants of FOXP2, the locus which seems to be correlated to elevated vocal and auditory capabilities across many vertebrate lineages. And, if it is correct that ~2.5% or so of modern human ancestry in Eurasia, and nearly ~10% in Papua, comes from “archaic” lineages, then I think that should reduce our estimates of how different these humans were from the Africans.

Therefore you can posit two stylized scenarios of contrasts between Neanderthals/modern humans and chimpanzees/modern humans. In one model the difference between the two comparisons is fundamentally of degree. Neanderthals and chimpanzees are still disjoint from modern humans. That is, there’s no overlap in the traits. But, Neanderthals are far closer to modern humans, as would be likely expected from the phylogenetic relationship. Another model though is that Neanderthals and modern humans did differ, but there was a great deal of overlap. This is a model with qualitative differences from that of chimpanzees vs. humans. If the second model is correct, and I think with all that we know from the Neanderthal genome project would should take it more seriously, then looking for disjoint pairwise differences in allele frequencies is not the way to go in understanding how the two human lineages diverged in phenotype.

In the second model, where there is a great deal of overlap, there is still a difference in the tails of the distribution. The idea I had in mind with my earlier post was that it is at these tails that the differences between Neanderthals and modern humans will be found when it comes to cultural differences. I think one might wonder where the Michelangelo or a Bachs of the Neanderthals were, but then one has to observe that the vast majority of modern humans are not Michelangelo or a Bachs! One of the primary indications of the transition to behavioral modernity is the proliferation of symbolism. But are we to presume that every member of an ancient Paleolithic tribe was equally capable of creative virtuosity? I think likely not. It could be that in fact the vast majority of “modern humans” are no different from all Neanderthals in the sort of things we might expect to be different across the two lineages. Rather, it may be that a small minority of modern humans crossed a particular threshold at the edge of the distribution of the phenotype, and when that transition was made the world was never the same.


Julius Caesar

I’m not proposing here that the victory of African humans ~50,000 years ago was due to artists. What I’m proposing is that at some point a critical mass of exceptional individuals arose. These individuals were possessed of peculiar characteristics, but instead of these characteristics making them outcasts, the qualities which they possessed were seen by their fellow humans as marks of greatness. In short, they were the children of gods among men.

Or perhaps demons. Men such as Alexander, Napoleon, and Hitler, were possessed of peculiar charisma, but whether they were good or evil is a matter of dispute and perspective. The point is not that they achieved greatness, but that they were the catalysts for a great number of events. As charismatic leaders they took collections of human beings, and turned them to their purpose. Individual humans became more than the sum of their parts, and for moments exhibited almost organismic levels of cohesion. Though the number one predictive variable in who won wars in the pre-modern world is the simple one of numbers, organization and structure also mattered. The Roman legion operating in a Testudo formation could beat off the attacks of more numerous barbarians who were physically more robust on a per person basis because the unit exhibited synergy, and translated cohesion into efficient collection action. This does not occur bottom up, but requires a personality type, a genius, to serve as the nexus or locus.

The model I have in mind then is one where the African humans faced up against their near relations, but not as one against one. Rather, under the guidance of charismatic leaders, Paleolithic megalomaniacs driven by fervid nightmares and irrational dreams, they ground through the many enemies who fought as sums of singulars as a cohesive social machine. It was not because they were superior on a per unit basis, but because they were superior on a per tribe basis, driven by individuals who turned the many to their own ambitions. With the lever of superior social organization the few moved the world, and swept over it. How many insane voyages were their east over the horizon from Sundaland before one tribe finally made landfall in Sahul? How many tribes perished in the ice of the far north, before some finally made it to Beringia? Why did humans look over the horizon, and venture out across the black waters? Perhaps just because they could. This answer is likely confusing and disquieting to many alive today, and perhaps it was disquieting to the more reasonable and level-headed “archaics” who were confronted with the zealous organizational insanity of the African humans who were rolling all opposition. But these insane individuals still move among us today, and they are still the objects of curiosity, fear, and adulation.

Is this a crazy model? Yes, somewhat. But is it really anymore crazy than the model that there is a mutation which can encapsulate all that differentiates man from beast-man? I think not.

  • http://webtrough.wordpress.com DW

    So lots of small differences, then?

    Is there a reason why your ‘great leader’ effect couldn’t also be caused by the single-mutation separation?

  • Al Cibiades

    Wow! Just, wow.

  • DK

    the 95-99% sequence level identity is thought by some to be perplexing. Therefore models have emerged which appeal to gene regulation and expression, or perhaps other forms of variation such as copy number, to clear up how it can be that chimpanzees and humans differ so much.

    Quite trivial. The more relevant and important number is this: 80% of proteins are different between humans and chimps. ( http://www.ncbi.nlm.nih.gov/pubmed/15716009 – that is, only 20% of proteins between the two species share identical sequence.) With proteins, a single amino acid change has a potential to dramatically affect the properties of the protein. And any amino acid change always has at least some effect on its properties. So while undoubtedly there are differences in gene expression control and copy numbers, they are strictly not required to explain existing phenotypic differences between chimps and humans – existing protein differences can easily be enough.

  • Sandra

    DK and his religious psycho-babble again.

  • Darkseid

    after watching people like Alex Honnold i don’t see how this could be wrong. some people just live for creativity and excitement or they go crazy. they can’t *not* be that way. others, like me, just sit back a watch them:)

  • RafeK

    Crazy thesis but no more crazy then the giant leap thesis and certainly intriguing.

    Personally I wonder if the spread of neo-africans wasn’t primarily demographically driven. Could behavioral modernity have been more or less possible in most hominims of the later Paleolithic, and the key was simply demographic? That is to say could the acceleration of change in technology and behavior have simply been an effect of the development of a sufficiently dense population to produce more innovators per generation in a given area. If so then Neo-africans could simply have been the first to cross this demographic threshold. Once they were ahead of the curve on behavioral modernity they simply swamped out the archaics, much in the same way that neolithic peoples swamped out paleolithic peoples and Europeans swamped out north american natives. Does there need to be specific smoking gun for the demographic expansion of neo africans or is it simply a particularly powerful example of a common pattern in our lineage?

  • John Roth

    @6 RafeK.

    I think population density has a lot to do with it; there is, after all, only a certain amount of cultural capital a single person is capable of holding and passing on to the next generations. That does, however, simply push the question back one level: what gave our ancestors the ability to have higher population densities? That’s not, after all, something that exceptional individuals would be likely to affect significantly.

  • marcel

    Why do you think it had to do with charisma and its effect on military or quasi-military organization? Why not instead either more widespread insight of the type useful for technological advance, or greater frequency of more extreme insight of this sort? Why not Thomas Edison rather than Alexander the Great?

  • S.J. Esposito

    I think this is the centerpiece of the post: “The point is not that they achieved greatness, but that they were the catalysts for a great number of events.”

    It doesn’t seem all that unreasonable to me that major events have shaped the histories of all hominids and the points where they may have intersected one another. If this is true, then, these major events were probably caused by individuals who were distinctly different from the masses.

    It’s something we’ve seen in the recent fiction of the film Rise of the Planet of the Apes; one chimp, Caesar, is able to catalyze a series of major events that, presumably, would change the paradigm of the ape-human relationship on earth. Of course, fiction is fiction, but there is a similarity, however slight it may be.

    Excellent, thought-provoking post.

  • jb

    Razib, when I look at your second graph, with the overlapping curves, what comes to my mind is not charisma, but general intelligence. I’m thinking here of the Smart Fraction theory of La Griffe du Lion, which posits that the wealth of nations is critically dependent upon the percentage of individuals above some IQ threshold.

    Couldn’t behavioral modernity work the same way? Low IQ populations like the Neanderthals would be behaviorally static, but once the average IQ of some population somewhere in the world rises high enough you get a critical mass of creative individuals, innovation takes off, population increases, and everyone else gets swamped. There probably would be at least some genes unique to this population, but, since intelligence seems to involve a large number of genes of low effect, those genes would be difficult to identify.

  • Mary

    The model you describe makes a lot of sense. One always wonders how the tails of the bell curve survived in paleo times. Ostracism and abandonment surely culled the crop of exceptional individuals. But by serendipitous happenstance some individuals were able to put forward new ideas and convince enough of their cohorts to go along with them instead of kill them. And as you suggest, this was a rare event. How many brilliant ideas and actual achievements died out when not enough others were able to pick up and carry forward. Critical mass of like thinkers would be required. We only have to look at our own world to see that changes in all aspects of human endeavor is made by inches with the vast majority being dragged reluctantly or unknowingly in the wake of the new. Allow me to express my admiration for your writing and gratitude for your sharing.

  • Matt (another)

    The “Smart Fraction” theory of homo sapiens modernity? I think like that theory, it’s probably true that shaping conditions generating more “great men” matter to some extent and drive change, changes in the bulk of the population also matter to another extent. We’ll find that both are important (not that I am exactly contradicting anything you are saying when I say this).

    Stepping away from “All ATMH are this; all archaic humans are this”, and considering alternatives, seems like a step in the right direction though, regardless. It doesn’t (as I understand it) work for differences in cultural production between present day populations of H. Sapiens, so why be that optimistic about it as application to the differences between H. Sapiens and other hominids?

  • http://psilocybelacrosse.bravehost.com Psilocrosse

    I suppose the rammifications of this theory in the modern age is that too many cooks have spoiled Africa’s soup. I suppose that we could go even further and say that too many Africans could spoil Europe’s and America’s soup as well.

  • http://www.huxley.net/bnw/ Mustapha Mond

    @8 “Why do you think it had to do with charisma and its effect on military or quasi-military organization? Why not instead either more widespread insight of the type useful for technological advance, or greater frequency of more extreme insight of this sort?”

    Unless I’m reading Razib incorrectly, I’m not sure he is saying that.

    I’m skeptical of any “great man” theories. Were exceptional generals even possible before the advent of agriculture? The best military strategists prior to that only had above average marginal utility for their individual clan. Hardly enough juice for a great leap forward.

    It was only the gradual incremental increase in agricultural efficiency by the many that set the stage for enough surplus to allow for the rise of Egypt and Sumer.

    Was Alexander all that exceptional vis a vis the Persian emperors that preceeded him? He was just following the strategic aims and plans of his father and out of necessity kept much of the infrastructure and administration of the Persian Empire intact in his conquest. Indeed, see Pierre Briant as to why Alexander might more rightly be seen as the last Persian Emperor rather than its conqueror per se.

    Would Michelangelo necessarily be put above Leonardo or Raphael had they (or others) been able to snag his commissions?

    How great even was Newton considering Leibniz’s simultaneous development of the calculus. In either case they were both certainly indebted to and built upon the work of their contemporaries and immediate predecessors of the scientific Enlightenment such as Kepler.

    As much as it pains me to say it, one must condlude objectively that if J.S. Bach had not composed (over considerable time) the Mass in B Minor there would have undoubtedly been a work of equal beauty created by another, after all great musical achievements continued well into the 19th Century.

    I find it more likely that these charismatic military leaders or individual geniuses are a common function of any population that they inevitably arise to focus the creative milieu created by the top 10% or 20% (smart fraction) of the population.

  • Alexander Pico

    What’s a leader without devoted followers? Thus, a corollary to this idea of charismatic leaders emerging from the tail of our distribution is the idea that the propensity to follow and believe in (and fight for) insane leaders must be even more commonly distributed in “modern humans.” Looking around today, that’s not so hard to believe, right? [and the fact that it's "not so hard to believe" just further makes the point! :) ]

  • John Smith

    Razib, where have you been over at bloggingheads.tv? I’ve missed seeing you over there.

  • Kirk

    If you made your normal distribution a skewed normal with a long right tail you would see a bigger difference (a smaller overlap) between two populations. If the right most population had even a small difference – say in fertility or trait X – this could make sudden, significant increases in the long tail. Since the long tail has smarter, crazier, more depressed, more schizo-affective, more autistic extremes AND they have an increased chance of appearing near each other so that they can trade artistic images between them – in other words… they are primed for intersection with black swans. This is where chaos makes for wide swings in outcomes for small changes in initial conditions. This is more than a just so story since we know the answer. The population on the right took over the biospere, bootstrapped the noosphere and invented computers that could do Monte Carlo simulations.

  • RafeK

    John Roth, my speculation is that neo-africans simply lived in an environment which was more biologically productive and which humans were better adapted to, Eurasians were at a disadvantage because there were at the edge of the habitable zone for humans with pre modern behavior.

    Alexander I think that is an interesting model it could be that there were not more leaders per say but rather there was change in the mean capacity to follow someone elses lead, Domestic animals relative to wild animals are far more tolerant of strangers, more docile and more biddable, I think human evolution has been been moving in a similar direction starting with behavioral modernity. With a more biddable general pop the few exceptional individuals were able to organize mass action more effectively.

  • http://blogs.discovermagazine.com/gnxp Razib Khan

    Is there a reason why your ‘great leader’ effect couldn’t also be caused by the single-mutation separation?

    only point here is that we might go wrong looking for a gene where all moderns and neanderthals differ.

    Why do you think it had to do with charisma and its effect on military or quasi-military organization?

    there is some evidence that modern humans had larger group sizes.

    Razib, when I look at your second graph, with the overlapping curves, what comes to my mind is not charisma, but general intelligence. I’m thinking here of the Smart Fraction theory of La Griffe du Lion, which posits that the wealth of nations is critically dependent upon the percentage of individuals above some IQ threshold.

    it could be a range of traits.

    John Roth, my speculation is that neo-africans simply lived in an environment which was more biologically productive and which humans were better adapted to, Eurasians were at a disadvantage because there were at the edge of the habitable zone for humans with pre modern behavior.

    the tropics are more productive, but that’s not usually captured by animals with larger masses. i’d be curious about differences in large animals on the ice age tundra vs. savanna.

  • Dwight E. Howell

    Many good points. The population density/life expectancy point has to be crucial to culture advancing beyond the most basic way of life. I don’t see this as being the product of a sudden major genetic innovation. Culture is also going to advance the most where the environment cooperates. That could even mean being hostile but in ways that people could deal with.

    “Great men” are only going to be able to use that mass to take over after innovation and adaption allows it to develop.

  • 4runner

    So– if a chimp Alexander the Great were born tomorrow, would he be able to organize other chimps into a phalanx and march to India?

    Of course not.

    The Macedonian population– with its ability to work for the common good– is the real lever.

  • http://washparkprophet.blogspot.com ohwilleke

    Elite innovation may be a factor, but elite innovation and leadership aren’t exclusive to humans. Great apes have lots of social structure and have been observed to have one member develop new food acquisition technology used by others in the group and continued to further generations. If is also seen in chimps, for example, and if that were such a huge factor, the fact that they’ve had much longer to innovate would have allowed them to accumulate more innovations and be more behaviorally similar than they are.

    Plasticity in thinking may be a bigger deal. The Oldowan, Mousterian, modern human cycle is suggestive of the notion that we are far less hard wired that our predecessor species. “Great men” can only be great if their followers are capable of doing things radically different than those who came before them. They may mold people to their ends, but equally important, the people have to be possible to mold.

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Gene Expression

This blog is about evolution, genetics, genomics and their interstices. Please beware that comments are aggressively moderated. Uncivil or churlish comments will likely get you banned immediately, so make any contribution count!

About Razib Khan

I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. In relation to nationality I'm a American Northwesterner, in politics I'm a reactionary, and as for religion I have none (I'm an atheist). If you want to know more, see the links at http://www.razib.com

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