After my post, Functions oh So Random, which comments on old arguments about contingency in evolutionary biology, a reader pointed me to an excellent feature in Nautilus, If the World Began Again, Would Life as We Know It Exist? It explores the question in greater depth, and reviews many of the contemporary players. The primary representative for the idea that evolutionary processes will tend to converge upon a finite set of specific adaptive peaks is still Simon Conway Morris, who seems to argue that experimental evolutionary results which indicate the likelihood of contingency just haven’t gone on long enough (I wonder, how many generations, Dr. Conway Morris?). It strikes me that Conway Morris is unlikely to ever be satisfied unless we discover life on another planet, which has the potential to falsify his model. But his comments probably did push me more toward the power of contingency, in particular:
Conway Morris believes that, over time, natural selection leads organisms to evolve a limited number of adaptations to the finite number of ecological niches on Earth. This causes unrelated organisms to gradually converge on similar body designs. “Organisms have to configure themselves to the realities of the physical, chemical, and also biological world,” he says. In Conway Morris’s view, these constraints make it all but inevitable that if the tape of life were replayed, evolution would eventually reproduce organisms similar to what we have today. If humans’ ape ancestors had not evolved big brains and the intelligence that goes with them, he believes that another branch of animals, such as dolphins or crows, might have, and filled the niche that we now occupy. Gould disagreed.
The idea that physics implies particular body plans strikes me as plausible. Here there seem to be limits to contingency. But the assertion that intelligence is in some way a niche is a jump too far for me, at least to an extent. More on that later. First, let’s note that it seems highly unlikely that organisms adapt to a niche which exists in a Platonic sense as a fixed idea in the firmament. Organisms evolve in the context of each other, adapting not only to the physical world, but inter and intra specific pressures. Ergo, the idea that sex persists among complex organisms despite its cost because of co-evolutionary pressures of infection by pathogens.* But when it comes to intelligent life forms we can extend the complexity further, because arguably one of the primary adaptive feedbacks of these organisms is going to be their own cultural production. In other words, one would have to also argue that cultural production itself exhibits some level of inevitable convergence upon a fitness peak.
But I don’t want to get carried away. Obviously there are some cultural forms which are not adaptive. Shaker obligate celibacy comes to mind. But the range of possibilities for cultural expression is still complex. And going back in time I think it is important to suggest that even if contingency rules over the macroscale, it may not be as powerful over a shorter timescale. A few years ago Luke Jostins produced the above figure to show that distinct hominin lineages, which we believe were genetically isolated by and large, nevertheless were all increasing in cranial capacity over the Pleistocene. We do not know why, but the chart suggests that there are some powerful common forces which can overcome phylogenetic divergence.
Ultimately though the argument about contingency is fascinating, it strikes me that it is not entirely scientific in its deepest level. It reminds me of an argument I encountered in Cultural Evolution: How Darwinian Theory Can Explain Human Culture and Synthesize the Social Sciences. Recounting the emergence of the neo-Darwinian synthesis in the 1930s and 1940s the author suggests that it took so long partly because geneticists and naturalists were focusing on different evolutionary scales (micro vs. macro) and utilizing unintelligible languages. Because of the discrete Mendelian nature of inheritance geneticists were skeptical of Darwinian gradualism in evolutionary process and phenotypic characteristics. While naturalists had difficulty conceiving of how isolated mutations could result in the panoply of diversity they saw around them. The conflict was resolved with the development of a formal language which could translate the two scales, population genetic theory. Population genetics illuminated quite elegantly how numerous genes of discrete effect could combine to produce quantitative traits and gradual evolutionary change, and, how low rates of mutation might nevertheless allow for rapid change on a geological scale through selection pressures. Without a formal language the two groups had to rely on intuition and kept talking past each other.
We’re at a similar juncture when it comes to nearly meta-scientific questions such as contingency. We can’t even know who is right until we know the right questions to ask. At that point the write up will be in Nature Reviews Genetics, rather that long popular science books or features.
* Obviously the physical world itself can be changed by biology. Oxygen producing bacteria totally reshaped the biosphere.