We're distantly related, you and I. Somewhere deep in the past, the two of us share a common ancestor. The same can be said for us and chimps. Or chimps and alligators. But what about alligators and sycamore trees? Or humans and Tyrannosaurus rex? Are you and I, as paleontology expert Brian Switek puts it, distant (distant, distant, distant) cousins of the great, tyrant lizard?
Yes. In fact, the concept that all life on Earth is related is a central tenet of Darwin's theory of evolution. Even organisms divided into Archaea, Bacteria and Eukarya, the three highest taxonomical ranks of life, share ancestors. The big question for scientists now is when that ancestor lived — and more importantly, how it lived. Here's what we know so far.
"Darwin was the first person to lay out this idea of a tree of life," explains Boston University evolutionary biologist Christopher Schneider in an interview with io9. "He presented evidence that any two things share a common ancestor at some point in the past, and proposed a mechanism — descent with modification — to explain how that happens."
Granted, the common ancestor shared by humans and chimps is different from the one shared by humans and T. rex, but go far enough back in time and you'll encounter a last universal common ancestor, or "LUCA," that is common to every living thing on Earth. Darwin referred pretty explicitly to LUCA, though he never referred to it as such. In his seminal work, On the Origin of Species, Darwin wrote:
Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.
One of the best ways to illustrate the concept of LUCA is with a Hillis Plot, a circular diagram that illustrates the evolutionary relationship between thousands of organisms across the tree of life — from the thickest, foundational branches, to the manifold offshoots that reside, twig-like, along the diagram's fringe. At the center is where one would find the universal common ancestor. Phylogenetic trees like the one above (click here to see a hi-res version of a much, much more detailed Hillis plot) can make it easier to understand the array of ramifying evolutionary connections between seemingly unrelated organisms, and can give rise to some surprising realizations.
"A friend of mine, Harry Green, wrote a great article called 'We are primates and we are fish' that's all about the importance of phylogeny and understanding comparative biology," says Schneider. What Green is saying is that, in ways that most people can wrap their heads around, we are primates. We share a common ancestor with all primates. "But we are also fish," explains Schneider, "in the sense that the most recent common ancestor of all land animals, all tetrapods, is a fish." He continues:
In fact, there are instances where we're more closely related to one fish than that fish is to another fish. So yes: we are primates, but we are also fish. And if you keep building back from this idea — we are vertebrates, we are chordates, we are animals, we are eukaryotes — eventually you reach a single common ancestor that was, presumably, the forbear to all life — whether it's Bacteria, Archaea or Eukarya.
Schneider explains that just about all evidence that researchers have been able to gather to date suggests that a universal common ancestor existed at some point between 3.5 and 3.8 billion years ago, and that it is a vastly more likely scenario than all life descending a single ancestor than from multiple ones. One of the more compelling studies to explore the likelihood of single vs. multiple common ancestors, Schneider explains, was published by researcher Douglas Theobald in Nature in 2010.
The study concludes that the likelihood of all life descending from a single universal ancestor is at least 102860 times more likely than a multiple ancestor scenario. When Theobald accounted for the possibility of horizontal gene transfer (the potential for sharing of genes between organisms in different lineages, a phenomenon believed by many to have been common among early species of bacteria and archaea), the odds of a single-ancestor vs. multi-ancestor scenario skyrocketed to 103489 times more probable.
Of course, the identity of a universal common ancestor is a lot less clear. "It's hard to make inferences about what that thing was or what it looked like or anything else," explains Schneider, "so in terms of the universal common ancestor of all currently living things, of course it had to be some single-celled prokaryotic organism, but beyond that it's hard to say much about it." He continues:
It certainly used DNA as a genetic material, but the ultimate issue in questions like these is: "Well, how far back do you want to go?" Do you want to go back to the origin of life? I think it gets very dicey. If you want to talk about the common ancestor of ALL extant living forms, then we're talking about, you know, some single celled prokaryote. But where did that come from, you know? The origin of life is really a region of very active investigation — there's just no clear-cut, "winning" hypothesis yet.