Evolutionary tree, Darwin’s notebook

Whether you like it or not, you are, in fact, my cousin. We can verify this through a couple of complementary lines of argument, one working forward in time, the other working backward. First, we know that all human beings comprise a single species, Homo sapiens. In 2005, Ian McDougall and colleagues reported in the journal ‘Nature’ that the oldest fossils belonging to our species were dated at 195,000 years old, and were discovered in Omo, Ethiopia. We all share this common point of origin as our lineages have intertwined and diverged over the last 8,000 to 9,000 generations or so.

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A second argument, borrowed from Bill Bryson, makes the same point by utilizing some basic math. Every generation we go back, we double the number of biological ancestors (2 parents, 4 grandparents, 8 great-grandparents, etc). This is 2ⁿ, where n is the number of generations. If you take out your calculator and go back 10 generations (2¹⁰), you’ll find we have 1,024 ‘great-grandparents.’ In 30 generations, we have 1,073,741,824. In just 80 generations, or only about 2,000 years, we have 1.2 x 10²⁴ ‘great-grandparents’ (which comes to roughly 1,200,000,000,000,000,000,000,000 ‘great grandparents’). However, we know this figure is a mathematical impossibility, as not that many people have ever lived. For comparison, the number of people currently on the planet is ‘only’ about 6,700,000,000, which is a drop in the bucket compared to 1.2 x 10²⁴. And keep in mind, this is only for a calculation done for 2,000 years; if we did the analysis for the entire 195,000 years that our species has been on the planet, then the number of direct ancestors would be even more ludicrous.

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Why the disparity then? The reason is simple- we all share ancestors through inbreeding. To some degree, everyone on the planet has some  inbreeding in their ancestral past. In a metaphoric sense then, we really are one big family. To my mind, the best example of this was reported in 2008, when genealogists discovered that even Barack Obama and Dick Cheney were 11^th cousins, while Obama and George W. Bush were 10^th cousins.

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Charles Darwin understood that this pattern went beyond the level of individuals to something deeper. The historians Adrian Desmond and James Moore argued in their recent book ‘Darwin’s Sacred Cause’ that Darwin’s family background and education instilled in him a disdain for slavery which greatly influenced his view that all human races comprised one large family. However, Darwin went one step further. Not only did all humans fall into one species, with all individuals sharing a common ancestor, but so did all species with each other. Each species, from finches to humans to orangutans to barnacles to pigeons to cabbages, were cousins on the tree of life.

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But we can go even one step further than this. Astrophysicists remind us that in the early history of the universe only the lighter elements existed (hydrogen, helium). All other elements were created in the nuclear furnaces of stars which compressed lighter elements into heavier ones. When those stars exploded, they dispersed their newly created elements, and it is from these that our planet, and our bodies, are made. Even more mind-boggling, as the physicist Lawrence Krauss said, is that the atoms in your right hand probably came from a different star than the ones in your left hand. One thought-provoking way to view this is that we are essentially a piece of the universe that has gained self-awareness and the ability to ponder itself. In Carl Sagan’s words, we are “star stuff contemplating star stuff.” Therefore, we are all connected to each other in a very real and fundamental sense. What’s cooler than that?

XXX