Saturday, 11 October 2008

Transitional fossils

It's thought that 99.9% of species that ever lived on this planet are no more. Extinct, dead, never to be seen again. But their existence is not without recognition. A few of the species are ancestors of creatures that are alive today. And a few other species are still to be found, preserved in the earth itself, buried by time only now for a species to examine and understand their significance. Fossils are but a brief glimpse of the past, yet they tell us so much about what life was like at certain periods in earth's history. Just think that 65 million years ago, the last of the dinosaurs were still hunting the plains of North America. Marsupials were already running rampant in Australia and South America. Birds were flourishing in the sky, cousins of the mighty land reptiles that were soon to be lost to time.

The missing links
Evolutionary theory predicts that not only would there be a fossil record, but the fossil record would contain ancestors of creatures that are alive today. The problem is that there is no way to tell if a species was a direct ancestor of the modern species. Take archaeopteryx, possibly the most famous transitional form. It provides a missing link between dinosaurs and birds. It has a reptilian tail, it has a reptilian beak. Yet it not only has wings but feathers too. Now this isn't the only dinosaur discovered with feathers. The velociraptor (as made famous in that brilliant Michael Crichton book and not-quite-so brilliant film) also had feathers, as did many of the Cretaceous predator dinosaurs. But despite archaeopteryx being a perfect candidate as a transitional form between dinosaurs and birds, there is absolutely no way of telling if it's a direct ancestor or a cousin of a ancestor.

So what can a scientist do? There's no way to satisfy the "show me just one transitional fossil" challenge as all fossils that are transitional might not be direct. This way a creationist can dismiss all claims of transitional forms. What Archaeopteryx shows is how evolution took place, it shows the gradual transition between reptilian and bird features. Likewise, while the fossils found that chronicle the evolution of the horse may not be the direct ancestors of the horse, they show just how the modern horse would have evolved from it's fox-like ancestors. The uncertainty over direct ancestry misses the entire point over what transitional fossils tell us.

A good example of the predictive nature of evolution is tiktaalik. Based on what we know about the emergence of vertebrates from water onto land, scientists are able to make predictions about when certain transitional species would be present. And thanks to the work of geologists, the age of rock formations is known. So when a group of palaeontologists went looking for a fossil that would show the transition between a certain type of fish with some amphibian-like features (Panderichthys) that lived 380MYA and early tetrapods that lived 365MYA, the place to look would be in rock around 375 million years old. Tiktaalik was found only a few years ago, and it's features are both of that of a fish and a tetrapod. It's the archaeopteryx of amphibians. Even if it's not a direct ancestor of all tetrapedal life, it would be a close ancestor. It's significance is again it can show how a transitional species would have evolved.

The other type of transitional forms are intermediary species. These are species that would have descended from transitional forms, retaining features common to both forms yet having other adaptive traits that neither form had before. The monotremes of Australia are great examples of this. Platypus still lay eggs, yet have mammary glands to feed their young. Yet the platypus is a highly specialised aquatic creature with adaptive traits common to neither reptiles or mammals. Study of the platypus can provide much information on the evolution of mammals. Likewise reptiles can provide a link between mammals and amphibians, amphibians between reptiles and fish. There are descendants of species now that provide much information on the way certain species evolved. It's a creationist lie that no transitional forms have been found, and it's an even bigger lie to assume that even in the absence of transitional fossils that creationism provides a coherent alternative.

Punctuated equilibrium
There is a story of life that studying the fossil record tells. Life came on gradually, it's about a 3.8 billion year process. At the beginning life was simple, single-celled organisms and simple forms. Then there is the Cambrian explosion around 550 million, where multicellular life and diversity start to appear. First there are invertebrates, then jawless vertebrates, then jawed vertebrates. The gradual emergent complexity of life is shown through the different strata. That change in life over time is indicative of the process that bore us. There are gaps though, so many gaps, fossilisation is a rare occurrence and only happens under the right conditions. Darwin concluded that the fossil record was incomplete and that over time gradual changes would be found. It seems on that account, Darwin was off the mark.

Of course evolution doesn't live or die on the accuracy of The Origin Of Species. Like all scientific concepts it changes over time as more evidence comes to light, and evolution has undergone many changes to the initial concepts that Darwin came up with. One of those is the idea of the stability of species. As Gould notes, the fossils of populations are stable entities for millions of years, the variation does not point to gradualism in the pure sense. His view is in essence a different from of gradualism, though one that fits the fossil record.

Evolution does seem a highly conservative process, the amount of data that is replicated each generation is staggering in accuracy. Natural selection does play a role in this, it may be that it's favoured for populations to stay in stable forms. It may be also that if a population is big enough, the gene-pool will prohibit change as mutations do not spread through the population. Mutations might not be practically useful, as many niches in nature are already filled. For whatever reason this occurs, the stability of populations in the fossil record is there for all to see.

The debate concerning punctuated equilibrium is indicative of the ever changing nature of science. The theory of evolution is not gospel, it changes as new information comes to light. There is still much to learn in biology, we have but scratched the surface of scientific understanding. The story of the fossil record shows that life on earth has changed over time, that it had a very simple beginning and it's only in the last 550 million years that life as we know it today even started to take place. Fossils are a glimpse at the past, they are pieces of a jigsaw puzzle without knowing what the final product is meant to look like. Life has evolved, just what processes cause that evolution are the question. The fossil record shows that life evolved, it just has input into how we figure out it all happened.

1 comment:

Jared said...

Well, the whole punctuated equilibrium view may also be a result of large numbers of a population being killed at about the same time relatively close to one another. This would give the appearance of far more homogeneous fossils than the populations were at any given time. For example, if we take a small subset of a population, we cannot extrapolate the appearance of the entire species.
In any event, nuclear and mitochondrial genetic evidence as well as a few long-term experiments make fossil evidence unnecessary, but beneficial.