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March 26, 2019

The arrow of evolution

Filed under: History,philosophy,science,Teleology — Razib Khan @ 11:46 pm
5,000 years of change

Most evolutionary biologists would agree with the contention that evolution has no long-term direction. In other words, evolutionary change is shaped by the contingencies and exigencies of the present set of circumstances, searching blindly through “adaptive space” for local optimal solutions. Just “good-enough.” For now.

In the realm of biology, this is illustrated by various “retrofits” one can see in form and function. The human vertebrate is a notoriously suboptimal feature, but it is the outcome of hundreds of millions of years of prior evolution when our ancestors were primarily quadrupeds. We make upright posture and bipedal locomotion work for us, but there’s a reason that prehistoric bipedal dinosaurs would likely run us down with ease. Jurassic Park isn’t all fantasy.

Upright hominins are utilizing new engineering technology which still has a lot of “bugs.”


Even the classic view of human evolution where short slouching creatures become upright moderns requires a rethinking. The reality is that Homo erectus was probably already as tall, on average, as modern human populations. And until contemporary times the tallest human populations known seem to date from the Pleistocene. Humans have been shrinking since their Paleolithic peak, with the 20th-century spike across the developed world reversing millennia of decline.

If evolution has a goal, oftentimes it looks like it hasn’t made up its mind.

The late paleontologist Stephen Jay Gould made a forceful case for the importance of chance and necessity, randomness, in the diversity of life on earth. In his book A Wonderful Life, he states:

The divine tape recorder holds a million scenarios, each perfectly sensible. Little quirks at the outset, occurring for no particular reason, unleash cascades of consequences that make a particular feature seem inevitable in retrospect. But the slightest early nudge contacts a different groove, and history veers into another plausible channel, diverging continually from its original pathway. The end results are so different, the initial perturbation so apparently trivial.

Gould’s understanding of evolution is at one extreme of the views about the role of randomness, and lack of purposiveness, of natural history. It should not be a surprise that his view is not universally held.

The researcher upon whose work his argument in A Wonderful Life was built, Simon Conway Morris, strongly disagreed with Gould’s interpretation of paleontology and natural history. In a series of books, he outlined the idea that evolution does have a broad directionality due to various forces, such as the necessity of streamlined body-plans for predators in the ocean. The fact that dolphins resemble ancient Mesozoic marine reptiles is obviously not a coincidence and illustrates that rewinding the tape only led to large-bodied land vertebrates converging upon the same body plan to adapt to the oceans.

Ichthyosaur from the Mesozoic

Richard Dawkins reiterates this argument in The Ancestor’s Tale. Dawkins also observes that over the billions of years that life has been present on earth, there seems to be a gradual, if sometimes halting, progression toward greater complexity. Most of our planet’s history was dominated by cyanobacteria and other prokaryotes, but more recently multicellular life forms have emerged to make up a much greater proportion of the biomass and dominate ecosystems. Despite mass extinctions such as the one at the end of the Permian or the event that resulted in the extinction of dinosaurs 60 million years ago, multicellular life is here to stay.

Now, over the last several million years humans have been expanding across the face of the earth and reshaping the landscape. While it is easy to dismiss humans as just another medium-sized megafauna, today we and our domesticates account for 96 percent of the mammalian biomass on planet earth!

Human occupation and seen through the lights we produce

This is amazing in light of the fact that tens of thousands of years ago it seems likely that our species numbered in the tens of thousands. Today we number in the billions. It is just our luck and happenstance?

Humans are wont to perceive themselves as the pinnacle for creation, so scientists are cautious and skeptical of presuming we are in fact something special. Much of modern science has been involved in the project of showing just how banal our place in the universe is. An extension of the Copernican project, which rejected the old idea that the earth was the center of the universe.

We’re on the edge of a spiral galaxy, on a small planet circling a modest G-class star. Our species has been present on this planet for a tiny fraction of its existence of 4 billion years. What are we in the grand scheme of things?

And yet, it is quite likely that we are the first intelligent species on this planet capable of exploiting its mineral resources, judging by the fact that there were so many exploitable surface deposits of coal and iron during our own industrialization. The emergence of humans was also likely a final push in the extinction of numerous megafauna at the end of the Pleistocene, which had persisted through many interglacials.

Humans were the first large placental mammal to arrive in Australia and New Zealand, eventually bringing a host of others in their wake. No other large mammal seems to have been able to occupy six of the seven continents before our own species. And this feat was achieved 15,000 years ago when we were small hunter-gatherer bands.

Today, the entire biogeography of the planet has been resculpted by us, from red deer in New Zealand to European earthworms in North America.

This is then the “Anthropocene.” An age when humanity holds the leash on the planet’s biosphere and engages in endeavors which will change the future of the planet, or endanger it….

Breaking the Malthusian trap!

The pessimistic outcomes for the planet due to the evolution of humans, an intelligent, social, and technological, species have been well aired. But what about the optimistic ones?

There are some thinkers who have long argued that evolution on this planet was almost certainly at some point going to produce intelligent technological life. The “encephalization” of large animals on this planet, the relative size of the brain in relation to the body, had been gradually increasing long before the emergence of the hominin lineage, which is well known to be defined by very large brains.

If organisms are biological machines, the machines have been getting more complicated over time.

In the 1950s a Catholic priest and paleontologist named Pierre Teilhard de Chardin promoted the idea that evolution was driving the universe toward a cosmic consciousness. Most scientists were very skeptical of such teleological thinking. That evolution had some ultimate purpose beyond optimizing local fitness. Some direction toward a final culmination. Today, they still remain very skeptical.

Decades later in the 1990s the science writer Robert Wright wrote Nonzero: The Logic of Human Destiny. In it, he argued that evolutionary processes were leading up to humans and that our species was advancing through “non-zero sum” interactions. We had broken the grip of Malthus and the standard carrying capacity charts of ecologists through cultural innovation.

Today, David Sloan Wilson, a mainstream evolutionary biologist, argues that selection pressures on human groups may have driven increases in altruism and prosocial behavior to such an extent that he now imagines that successful completion of the Darwinian project may lead exactly to some sort of global consciousness, as envisaged by de Chardin. A mind which is able to optimize fitness at the level of the whole planet.

Of course, even David Sloan Wilson is not speaking in terms of inevitabilities. The Anthropocene is a dangerous time for the long-term time horizon of the planet’s ecosystems. The evolution of modern humans and their cultural creativity has unleashed a Pandora’s box of problems. But Wilson and his fellow-travelers seem to be suggesting that this is not just chance and necessity. That the earth has two possible final outcomes, one dire, and one nearly utopian.

And remember, opening Pandora’s box also unleashed “hope” upon the world.

The arrow of evolution was originally published in Insitome on Medium, where people are continuing the conversation by highlighting and responding to this story.

November 24, 2010

The inevitable social brain

ResearchBlogging.orgOne of the most persistent debates about the process of evolution is whether it exhibits directionality or inevitability. This is not limited to a biological context; Marxist thinkers long promoted a model of long-term social determinism whereby human groups progressed through a sequence of modes of production. Such an assumption is not limited to Marxists. William H. McNeill observes the trend toward greater complexity and robusticity of civilization in The Human Web, while Ray Huang documents the same on a smaller scale in China: A Macrohistory. A superficial familiarity with the dynastic cycles which recurred over the history of Imperial China immediately yields the observation that the interregnums between distinct Mandates of Heaven became progressively less chaotic and lengthy. But set against this larger trend are the small cycles of rise and fall and rise. Consider the complexity and economies of scale of the late Roman Empire, whose crash in material terms is copiously documented in The Fall of Rome: And the End of Civilization. It is arguable that it took nearly eight centuries for European civilization to match the vigor and sophistication of the Roman Empire after its collapse as a unitary entity in the 5th century (though some claim that Europeans did not match Roman civilization until the early modern period, after the Renaissance).

It is natural and unsurprising that the same sort of disputes which have plagued the scholarship of human history are also endemic to a historical science like evolutionary biology. Stephen Jay Gould famously asserted that evolutionary outcomes are highly contingent. Richard Dawkins disagrees. Here is a passage from The Ancestor’s Tale:

…I have long wondered whether the hectoring orthodoxy of contingency might have gone too far. My review of Gould’s Full House (reprinted in A Devil’s Chaplain) defended the popular notion of progress in evolution: not progress towards humanity – Darwin forend! – but progress in directions that are at least predictable enough to justify the word. As I shall argue in a moment, the cumulative build-up of compelx adaptations like eyes strongly suggest a version of progress – especially when coupled in imagination with of the wonderful products of convergent evolution.

Credit: Luke Jostins
Credit: Luke Jostins

One of those wonderful products is the large and complex brains of animals. Large brains are found in a disparate range of taxa. Among the vertebrates both mammals and birds have relatively large brains. Among the invertebrates the octopus, squid and cuttlefish are rather brainy. The figure to the right is from Luke Jostins, and illustrates the loess curve of best fit with a scatter plot of brain size by time for a large number of fossils. The data set is constrained to hominins, humans and their ancestors. As you can see there is a general trend toward increase cranial capacities across all the human populations. Neandertals famously were large-brained, but they exhibited the same secular increase in cranial capacity as African Homo. On the scale of Pleistocene Homo and their brains the idea of the supreme importance of contingency seems ludicrous. Some common factor was driving the encephalization of humans and their near relations over the past two million years. This strikes me as very strange, as the brain is metabolically expensive, and there are plenty of species with barely a brain which are highly successful. H. floresiensis may be a human instance of this truism.

But what about the larger macroevolutionary pattern? Is there a trend toward larger brain sizes in general, of which primates, and humans in particular, are just the most extreme manifestation? Some natural historians have argued that there is such a trend. But, there is a question as to whether increased brain size is simply a function of allometry, the pattern where different body parts and organs tend to correlate together in size, but also shift in ratio with scale. The nature of physics means that very large organisms have to be more robust because their mass increases far faster than their surface area. By taking the aggregate relationship between body size and brain size, and examining the species which deviate above or below the trend line, one can generate an encephalization quotient. Humans, for example, have a brain which is inordinately large for our body size.

And yet there are immediate problems looking at relationships between body and brain size, and inferring expectations. Different species and taxa are not interchangeable in very fundamental ways, and so a summary statistic or trend may obscure many fine-grained details. A new paper in PNAS focuses specifically on various mammalian taxa, corrects for phylogenetics, and also relates encephalization quotient by taxa to the proportion of social animals within each taxon. Encephalization is not a universal macroevolutionary phenomenon in mammals but is associated with sociality:

Evolutionary encephalization, or increasing brain size relative to body size, is assumed to be a general phenomenon in mammals. However, despite extensive evidence for variation in both absolute and relative brain size in extant species, there have been no explicit tests of patterns of brain size change over evolutionary time. Instead, allometric relationships between brain size and body size have been used as a proxy for evolutionary change, despite the validity of this approach being widely questioned. Here we relate brain size to appearance time for 511 fossil and extant mammalian species to test for temporal changes in relative brain size over time. We show that there is wide variation across groups in encephalization slopes across groups and that encephalization is not universal in mammals. We also find that temporal changes in brain size are not associated with allometric relationships between brain and body size. Furthermore, encephalization trends are associated with sociality in extant species. These findings test a major underlying assumption about the pattern and process of mammalian brain evolution and highlight the role sociality may play in driving the evolution of large brains.

A key point is that the authors introduce time as an independent variable, so they are assessing encephalization over the history of the taxon. This is clearly relevant for humans, but may be so for other mammalian lineages. The table and figures below show the encephalization slope generated by using time and body size as the predictors and brain size as the dependent variable. A positive slope means that brain size is increasing over time.

Two major points:

- Note that the slope is sensitive to the level of taxon one is examining. A closer focus tends to show more variance between taxa. So, for example, humans distort the value for primates in general. Bracketing out anthropoids paints a more extreme picture of encephalization, a higher slope. In contrast, the lemurs and their relatives exhibit less encephalization over time.

- The correlation between proportion of species which exhibit sociality and encephalization of the taxon is strong. From the text:

Encephalization slopes were correlated with both the proportion of species with stable groups (order R = 0.92, P = 0.005, n = 6; suborder R = 0.767, P = 0.008, n = 9; Fig. 2 A and B) and the proportion in either facultative or stable social groups (order R = 0.804, P = 0.027, n = 6; suborder R = 0.63, P = 0.04, n = 9).

The last figure makes it is clear that the correlations are high, so the specific values should not be surprising. Don’t believe these specific figures too much, how one arranges the data set or categorizes may have a large effect on the p-value. But the overall relationship seems robust.

A highly encephalized “alien”

What to think of all of this? If you don’t know, one of the authors of the paper, Robin Dunbar, has been arguing for the prime importance of social structure in driving brain evolution among humans for nearly twenty years. The relationship is laid out in his book Grooming, Gossip, and the Evolution of Language. Robin Dunbar is also the originator of the eponymous Dunbar’s number, which argues that real human social groups bound together by interpersonal familiarity have an upper limit of 150-200. He argues that this number arises because of the computational limits of our “wetware,” our neocortex. Those limits presumably being a function of biophysical constraints.

One interesting fact though is that the median cranial capacity of our species seems to have peaked around one hundred thousand years ago. The average human today has a smaller brain than the average human alive during the Last Glacial Maximum! (see this old post from Panda’s Thumb, it’s evident in the charts) This may be simply due to smaller body sizes in general after the Ice Age. Or, it may be due to the possibility that social changes with the rise of agriculture required less brain power.

Ultimately if Dunbar and his colleagues are correct, if social structure is the most powerful variate in explaining differences in brain size when controlling for phylogenetics and body size, then in some ways it is surprising to me. After all, it does not seem that ants have particularly large brains, despite being extremely social and highly successful. Clearly the hymenoptera and other social insects operate on different principles from mammals. Instead of
developing “hive minds,” it seems as if in mammals greater social structure entails greater cognitive structure.

Citation: Susanne Shultz, & Robin Dunbar (2010). Encephalization is not a universal macroevolutionary phenomenon in mammals but is associated with sociality PNAS : 10.1073/pnas.1005246107

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