The Next Big Thing in Evolution
In High School, we learned that the first- and second derivatives of position are velocity and acceleration. But what is the third derivative? It turns out to be “jerk,” which makes intuitive sense. When you step on the gas in an automobile, your car suddenly “jerks” from zero acceleration to around 3 m/s². What about the fourth derivative? Apparently, this is called “jounce,” which I can’t find an intuition for, but it could maybe describe that brief period when you’re putting your phone away and turning the key in the ignition.
Is there a similar set of intuitions for the “speed” of evolution? According to paleobiologist, Philip Gingerich, asking about the speed of evolution is like asking about the rate of change of human body temperature. The answer depends on the context — or the window — over which you’re measuring change. Your body temperature may change quickly in the first few minutes you step on a treadmill, for example, but then appear to change slowly when measured from morning to evening.
Apparently the fossil record “moves” like this, like body temperature, something biologists have known for over 40 years. Prior fossil records showed that morphological changes, like height, move gradually, like the famous linear cartoon of human evolution:
However, at shorter time scales, which are easier to measure with modern instruments, the changes are more abrupt. To make it more real, consider the difference in nose shape between you and your parents. If an alien in the far future only had the fossils of you and your parents’ generation to examine, it would seem like nose shape “evolved” really fast in that one generation. And yet, the difference between your nose shape and your great-great-x20-parents’ would roughly be the same, making the rate of evolution seem more gradual. The rate of evolution depends on time.
When the concept of “time-dependent rate of evolution” first reared its head, it apparently ruffled some feathers. Stephen Jay Gould was the first to respond in 1984, calling this inverse relationship between rates and their denominators a “psychological and mathematical artifact.” (If you recall, this is the same Stephen Jay Gould who proposed his own revolutionary theory of evolution called “punctuated equilibrium.”) In 2003, the interdisciplinary biologist Peter Roopnarine called the inverse relationship between rate and timescale a “mathematical artifact predictable on the basis of the behavior of random walks.”
What isn’t controversial, though, is the shape of the fossil record, which may be better described as “wobbly,” a term I lifted from New Zealand philosopher Kim Sterhly. At the highest zoom level, the rate of evolution appears like a step-function, as how Stephen Jay Gould described it, which is now consensus. Species stay in a relatively static equilibrium with their niche, until bam, a disruption occurs, and the species either splits, drifts, or goes extinct.
Wobbling, on the other hand, proposes that a lot of change happens during periods of stasis. When applied to human brain size, the average size is 1,500 cubic centimeters (cc), whereas our ancestor of 2 million years ago, Homo Erectus, had brain sizes around 850 cc. According to gradualism, humans had to have added about 250,000 neurons per generation, or about 12,500 neurons per year, which is the number of neurons that fits into the period at the end of this sentence.¹ If we take a punctuated equilibrium approach, the brain size must have gone through “jerks” or punctuations, adding something like 125,000 neurons per year during short bursts. But if we take a third approach, of wobbly stasis, then neuron counts fluctuate constantly, from generation to generation, much like the change in nose shape between you and your parents.
Could these changes explain the Flynn effect, which is the observed phenomenon whereby IQ seems to advance 0.2 points per year?² Probably not. Because, wouldn’t you know it, brain sizes have actually been shrinking! The BBC recounts around six researchers with six different estimates (and theories) about these brain size changes. Did the shrinking occur because the transition to farming deprived us of essential minerals and nutrients 10,000 years ago? Did brain sizes get smaller because the spontaneous invention of language gave us a jump in cognitive efficiency 100,000 years ago? It’s hard to say which theory is correct. As cognitive scientist Jeff Morgan Stibel observed, there aren’t any declines in that data that aren’t proceeded by reversals later. The more fossils you collect, the more stories you can tell. As the data increases, so does the number of theories, and until we see the broader pattern, our understanding of evolution will continue to wobble.
[1]:
[2]: “For example, a study published in the year 2009 found that British children’s average scores on the Raven’s Progressive Matrices test rose by 14 IQ points from 1942 to 2008”: 14/(2008–1942)=0.21
