Perhaps the most damning criticism of Charles Darwin’s theory of evolution by natural selection was based on fossils. If organisms evolve such that their descendants adopt radically different forms than their ancestors, why don’t we see evidence of this in the fossil record? As if on cue, two years after Darwin outlined his theory in On the Origin of Species, scientists uncovered Archaeopteryx, a beautifully preserved animal seeming to have mixture of reptilian and bird-like characteristics. And so the first ‘missing link’, or transitional fossil, was discovered.
150 years later, skeptics of evolutionary theory will frequently say that Archaeopteryx does not actually qualify as a transitional fossil , that there are practically no other putative transitional fossils to speak of , fossils are always found fully formed with no realistic progenitors , and the fossil record is complete, so we should not expect to find any other alleged missing links .
I think that the first point is the most important and is really worth considering: what constitutes a convincing transitional fossil? The first quality such a specimen must absolutely possess is the presence of characteristics intermediate between two other fossil forms. Archaeopteryx most certainly qualifies on this point. For example, birds have feathers and wings, whereas modern reptiles do not, and modern reptiles have tails, teeth and claws, whereas modern birds do not. Archaeopteryx has all of these traits, proving that it is transitional in form.
But having an intermediate form does not mean that Archaeopteryx descended from reptiles and gave rise to birds. Take for instance, the platypus. The platypus has hair and produces milk for its young like mammals, has a beak like a duck, produces venom like a snake, and lays eggs like a lizard. Does this make the platypus a transitional animal that connects reptiles and birds to mammals?
This is where the next piece of critical information comes in: the dimension of time. It isn’t enough to look like a medley of organisms. Rather there has to be a logical chronological sequence to their appearances. Geologists use a variety of information, including the clock-like radioactive decay of atoms, to estimate the ages of rocks. If Archaeopteryx appears in rocks that are more recent than those of the earliest reptiles, but also pre-dates the appearance of the earliest birds, then it constitutes a specimen consistent with it being a transitional fossil. (spoiler alert: it does)
And this is a testable, refutable hypothesis. If tomorrow scientists provide insurmountable evidence that modern bird fossils predate Archaeopteryx and/or reptiles, we can no longer say that Archaeopteryx meets the transitional fossil criteria.
But why aren’t there more fossils like Archaeopteryx? There are, but certainly not the millions and millions that must have existed if evolutionary theory is correct. So why haven’t we found them?
The simple answer is bias. There is a whole field of research (taphonomy) that seeks to understand when, where and how fossils are formed, and to be blunt, we think we’re missing a lot of information. The general idea is that you have to die in the right place, at the right time, with the right conditions in order to preserve information to last millions of years. The fossils that researchers tend to find are almost always extremely fragmentary and limited. For instance, fossil teeth are extremely abundant. Fossil brains and skin? Not so much. Whole organisms? We wish!
Then there’s a more human problem: paleontologists tend to focus on the best fossil-bearing rocks that they’ve discovered so far, yet even those are limited. Not only do the fossils have to actually land in the right place and the right time, but the earth has to shift and erode in such a way to expose the fossils. How many fossils might be under your feet right now, but you’ll never know because you’re never going to dig under your house? How many fossils are buried in the sea? Under antarctic ice? What are we missing out on that we could find only if we could CT scan the entire earth?
So the fact that there are only so many fossils we can find, and we only find them at a certain rate, and we usually only get to look at parts of bones and not the squishy parts of the bodies that presumably also were evolving, how much information are we missing out on here?
Which brings us to the third common point: if the fossil record is complete, how could we ever find any other transitional fossils? I’m not sure who first conveyed this idea, but boy were they wrong. As just one simple example, consider the fossil record of Mesozoic mammals, i.e. mammals that are found in rocks that date between 251 and 66 million years ago. Between 1830 and 1979, researchers found 116 general kinds of mammals that date to this era. Between 1979 and 2007, that number climbed to ~310 . Far from being complete, it appears that paleontologists are just getting started!
You can bet that as paleontologists keep digging, researchers will uncover more putative transitional fossils, many of which I document here in this blog. Keep an eye out for them, and ask yourself: do these fossils meet the criteria being transitional?