Over my last few blogs, I have found myself going backwards in time. I have written about the evolution of the pangolin. I have explored the story of the African wild dog. And while researching these animals, I kept coming across the same fascinating idea: that the animals we see around us today are the products of millions of years of evolutionary change.

But then a question started bothering me.

If evolution is a process of one form gradually becoming another, why don’t we have a fossil record showing the whole process?

Why do we find an ancient species, then another species that appears somewhat different, and then another? Where are all the fossils showing every tiny step in between? Where is the fossil of the animal that was 50% of the way between the old species and the new one? And then the next one? It almost seems as though evolution has left us a collection of photographs when what we really want is the movie. The answer is far more interesting than simply saying that the fossils are missing.

A fossil is a moment in time

The first thing to remember is just how extraordinarily unlikely it is for an animal to become a fossil. When an animal dies in the African bush, the normal outcome is not fossilisation. It is disappearance. Scavengers arrive. Vultures strip the flesh. Hyenas crush the bones. Insects consume what remains. Microorganisms break down the tissues. Rain moves material. Rivers transport sediment. Plants grow through the remains. Eventually, almost everything is gone.

A hippo carcass is completely covered as the vultures fight amongst each other for their share.

For fossilisation to occur, very specific circumstances generally have to come together. The remains need to be preserved before they are completely destroyed, often through rapid burial in sediment, and then survive geological processes for thousands or millions of years. So when we look at the fossil record, we are not looking at a complete archive of everything that ever lived. We are looking at the tiny fraction that happened to leave evidence behind.

Evolution isn’t a ladder

There is another reason our expectations can be misleading. We often imagine evolution as a straight line: Species A becomes Species B, then C and so on. But evolution doesn’t work like a ladder. It is much more like a branching tree. A population can split into separate populations. Those populations can experience different environments, different pressures and different mutations. Over generations, they can become increasingly different. Some branches eventually disappear. Others survive. And some branches give rise to further branches. This means that when we discover two different fossil species, they aren’t necessarily simply “before” and “after” versions of one another. They may be related branches of a much larger evolutionary tree.

Source

Evolution happening in shorter “bursts”?

Now that we understand just how unlikely it is for an animal to become a fossil, we can begin to understand another reason why the fossil record can appear to show us evolution in stages rather than as a continuous process. Evolution does not always proceed at the same speed. Imagine a population of animals living in a relatively stable environment. If the conditions remain similar for thousands of generations, there may be relatively little pressure for the population to change dramatically. Small evolutionary changes can still occur, but the overall form of the animal may remain relatively stable.

Now imagine that the environment changes. The climate becomes considerably drier. A forest retreats and grassland replaces it. A new predator arrives. Food becomes scarce. A population becomes isolated from the rest of its species. Suddenly, the animals are facing a different set of challenges. Individuals with traits that better suit those new conditions are more likely to survive and reproduce. Over many generations, those characteristics can become increasingly common within the population. In these circumstances, evolutionary change can occur relatively rapidly compared with periods when the external environment changes little. This doesn’t mean that an animal suddenly changes into a new species. Evolution is still happening through generations of genetic variation and natural selection. But if a population experiences strong environmental pressure for a prolonged period, substantial evolutionary change can potentially accumulate over a comparatively short period of geological time. And this creates an interesting problem for the fossil record.

Imagine that a species existed for two million years, but underwent a relatively rapid period of evolutionary change over perhaps a few tens of thousands of years. We might have millions of years of fossils showing the older form, followed by fossils showing the newer form, while the relatively short period in which the major changes occurred is poorly represented. It would almost look as though the old species disappeared and the new species simply appeared. But the transition may have happened over thousands of generations.

And even if the transitional animals did exist, the chances of any individual one of them becoming fossilised, surviving millions of years, and then eventually being discovered are extraordinarily small. This is one reason the fossil record can give us something that looks more like a series of snapshots than a continuous film. There are enormous gaps between the photographs, and we will probably never recover every missing image. But by studying the photographs we do have, comparing them with living animals and understanding the environments in which they lived, we can begin to reconstruct the story.

And perhaps that is the most remarkable thing about evolution. The story of life has been happening for billions of years, yet we have only just begun to piece together the photographs.