But evolution converges!

Stephen Jay Gould became famous in part for his book Wonderful Life: The Burgess Shale and the Nature of History. By examining the strange creatures in the Burgess Shale formation Gould makes the case that evolution is a highly contingent process, and that if you reran the experiment of life what we’d see might be very different from what we have now.

But the scientist whose study of the formation that inspired Gould’s interpretation, Simon Conway Morris, had very different views. Though it can sometimes be churlish, his rebuttal can be found in The Crucible of Creation: The Burgess Shale and the Rise of Animals. Simony Conway Morris does not believe that contingency is nearly as powerful a force as Gould would have you believe. And his viewpoints are influential. Richard Dawkins leaned on him to make the case for convergence in evolution in The Ancestor’s Tale.

This crossed my mind when reading Carl Zimmer’s new column, When Dinosaurs Ruled the Earth, Mammals Took to the Skies:

Today, placental mammals like flying squirrels and marsupials like sugar gliders travel through the air from tree to tree. But Volaticotherium belonged to a different lineage and independently evolved the ability to glide.

They were not the only mammals to do so, it turns out. Dr. Luo and his colleagues have now discovered at least two other species of gliding mammals from China, which they described in the journal Nature.

Dr. Meng said that the growing number of fossil gliders showed that many different kinds of mammals followed the same evolutionary path. “They did their own experiments,” he said.

This ultimately comes down to physics. There are only so many ways you can make an organize that flies or glides. Mammals come to the table with a general body plan, and that can be modified only so many different ways.

This is not a foolproof point of datum in favor of convergence as opposed to contingency. Frankly these are often vague verbal arguments which are hard to refute or confirm. And even molecular evolutionary analyses come to different conclusions. It may be that we are asking the wrong question. But, it does suggest that evolution may work in a much narrower range of parameters as time progresses because of the winnowing power of selection.

What determines the rate of evolution


The tweet above from Wiley relates to a paper, Polygamy slows down population divergence in shorebirds. It’s a cool paper. I tweeted it. But does it relate to the “rate of evolution”?

There’s no definitive answer to this question. Different people will have different answers, as it was evident on Twitter. For me my surprise was due to my definition for what evolution is: change is allele frequencies over time. This is far more fundamental than speciation. But then I don’t think much about speciation.

Some people brought up divergence. But divergence for me is not necessary, a population could remain unitary but exhibit large allele frequency changes. Then again, if you study phylogenetics on a macroevolutionary scale, as most people who study phylogenetics do, then you would focus on divergence.

The logic of human destiny was inevitable 1 million years ago

Robert Wright’s best book, Nonzero: The Logic of Human Destiny, was published nearly 20 years ago. At the time I was moderately skeptical of his thesis. It was too teleological for my tastes. And, it does pander to a bias in human psychology whereby we look to find meaning in the universe.

But this is 2017, and I have somewhat different views.

In the year 2000 I broadly accepted the thesis outlined a few years later in The Dawn of Human Culture. That our species, our humanity, evolved and emerged in rapid sequence, likely due to biological changes of a radical kind, ~50,000 years ago. This is the thesis of the “great leap forward” of behavioral modernity.

Today I have come closer to models proposed by Michael Tomasello in The Cultural Origins of Human Cognition and Terrence Deacon in The Symbolic Species: The Co-evolution of Language and the Brain. Rather than a punctuated event, an instance in geological time, humanity as we understand it was a gradual process, driven by general dynamics and evolutionary feedback loops.

The conceit at the heart of Robert J. Sawyer’s often overly preachy Neanderthal Parallax series, that if our own lineage went extinct but theirs did not they would have created a technological civilization, is I think in the main correct. It may not be entirely coincidental that the hyper-drive cultural flexibility of African modern humans evolved in African modern humans first. There may have been sufficient biological differences to enable this to be likely. But I believe that if African modern humans were removed from the picture Neanderthals would have “caught up” and been positioned to begin the trajectory we find ourselves in during the current Holocene inter-glacial.

Luke Jostins’ figure showing across board encephalization

The data indicate that all human lineages were subject to increased encephalization. That process trailed off ~200,000 years ago, but it illustrates the general evolutionary pressures, ratchets, or evolutionary “logic”, that applied to all of them. Overall there were some general trends in the hominin lineage that began to characterized us about a million years ago. We pushed into new territory. Our rate of cultural change seems to gradually increased across our whole range.

One of the major holy grails I see now and then in human evolutionary genetics is to find “the gene that made us human.” The scramble is definitely on now that more and more whole genome sequences from ancient hominins are coming online. But I don’t think there will be such gene ever found. There isn’t “a gene,” but a broad set of genes which were gradually selected upon in the process of making us human.

In the lingo, it wasn’t just a hard sweep from a de novo mutation. It was as much, or even more, soft sweeps from standing variation.

When the gods come crashing down

Sometimes the old gods slowly fade into oblivion. Contrary to popular perception this seems likely the case for ancient paganism. The conversion of Constantine to the Christian religion began the process of a hand-off and the commanding heights of classical culture that took over a century to complete. There were punctuating moments, such as the apostasy of Julian in the 360s, or the mostly symbolic ban on public paganism by Theodosius in the 390s (the Serapeum was destroyed by a vigilante mob). But pagans in the form of the Neoplatonic school persisted into the 6th century, while elite pagans such as Marcellinus maintained power and influence deep into the second half of the 5th century.

Call this “normal” cultural evolution. Antiquity evolved from being predominantly pagan to predominantly Christian (though a small cultured pagan minority persisted even until the Islamic conquest in the Near East, such as the Sabians of Haran).

The Reformation period was different. In a single generation one thousand years of a coherent and unified Western Christian ideology collapsed, and was replaced by something very different.

Note here that I said Western Christian ideology. The reality is that Western Christianity was never as unified or coherent as Western Christians themselves envisaged themselves to be (or aspired to be). There were episodes of hostility between particular kingdoms and the Roman papacy. Heresies such as that of the Cathars, and popular revolts with a religious tinge such as that of the Hussites. And finally, there were periods of multiple popes, which undermined the credibility of the institution of the Church in the medieval period.

But all this pales next to the magnitude and scope of the revolt against the establishment of the Western Christian church that occurred in the 1520s. Martin Luther went from being a Christian cleric within the established Church to declaring the pope the anti-Christian! Previously devout peasants in Switzerland turned on the relics and churches which they had only recently venerated, and engaged in mob iconoclasm. Whereas monarchs, such as Henry IV, ultimately compromised with the clerical estate (or, submitted), Henry VIII of England managed to destroy or subordinate the institutions of the church to his own will and pleasure.

There are many theories for why the Reformation occurred when it did. Some of them are rooted in technology, in particular the printing press. Others point to the development of proto-national identities, such as the rise of German nationalism and its leveraging by Luther against his “Roman” persecutors.

These specific issues are not interesting to me. Rather, what they point out to us that there can be cultural revolutions that occur very rapidly. One can point to the pacific post-World War II Japanese, and contrast them with the militaristic Japanese of the first half of the 20th century. Or the shift of Russia from being a conservative autocracy in the 1910s to a revolutionary society in the 1920s. But these are modern events, and moderns are liable to suggest that our own epoch is sui generis in these sorts of turnovers of values. But the Reformation shows that revolutionary changes in whole societies can occur rather rapidly even in a pre-modern context.

In other words, cultural revolution is not a derived characteristic of our species, but perhaps a very old one. The rapid expansion of the Austronesians. Or the radiation of non-African humanity. These come out of a vacuum, a cultural-demographic analog to the inflationary universe. But given enough time perhaps our species is simply subject to these sorts of explosions of creative change and innovation.