Jan 29 2009

Dinosaur Fossils and Evolution

Leading up to the 200th anniversary of Darwin’s birth on February 12th, and the 150th anniversary of the publication of the Origin of Species, I will be covering more evolution-related news items. It has long been a favorite scientific topic of mine anyway, so it’s a good enough excuse to focus on evolution.

One of the strongest lines of evidence for the fact that life on earth arose through evolution is the fossil record. I do not think this is the strongest line of evidence, not because it is weak but because the genetic evidence is so remarkably strong. Statistically speaking, the genetic evidence speaks to common descent through branching speciation of all life on earth to such a degree that it approaches certainty.

But the fossil evidence is also impressive, and much more visceral – walking into a museum full of gigantic fossil dinosaurs (or at least their casts) has a coolness factor with which a string of base pairs cannot compete.

Creationists, of course, deny the implications of the fossil evidence. At the extreme end are some young-earth creationists who claim that Satan put the fossils there to test our faith. That unfalsifiable notion is not worth further comment.

The less extreme evolution-deniers simply fail to grasp the history of science and the methods by which we test scientific theories. When Darwin published The Origin of Species in 1859 the fossil record was extremely sparse. Geologists had discovered enough fossils to see that different geological layers tended to contain different assortments of fossil species. This is what led originally to the notion that life has changed through geological time.   Fossils led to the evolution hypothesis even before Darwin.

But the fossil record that Darwin had access to was more gaps than anything else – provocative specimens scattered about the history of life without any patterns yet emerging, like a vast jigsaw puzzle with only a few pieces present, and with no picture as a guide to where the pieces went.

The theory of evolution made a very strong prediction, one that could potential falsify evolutionary theory completely. Evolution depends upon common descent – that all life is related through a series of branching events. Evolution therefore predicts that as we discover more fossils (more pieces to the puzzle) the picture that will emerge will be one of branching descent. Evolution requires this.

Branching descent has two main components – morphology and temporal sequence. The sequence of fossils should show morphological relationships in a branching pattern. One branch will have a set of morphological features that define the branch, and within that branch species will emerge with variations on those features. Occasionally a branch may give rise to a significant morphological change leading to a new branch.

In other words, evolution predicts that as we lay out fossil species morphologically they will fit into a branching pattern of relatedness. We will not see, however, random morphological features without a clear pattern. We will not see impossible hybrids, or entirely new complex structures arising without plausible antecedents, or the same specific derived characteristics in unrelated lines.

Further, the morphological branching pattern must simultaneously exist in a proper temporal sequence in the fossil record. Ancestors must come before descendants. We won’t find horses amid Cambrian fossils.

Over time these two related predictions of evolution have been validated spectacularly, and increasingly. The more fossils we find, the better they fit into a morphological and temporal sequence compatible with evolution.  Gaps are slowly filled, and we find generally what we expect to find. We have not found the equivalent of a horse in Cambrian strata. The fossil record has created thousands of opportunities to falsify evolution, and evolution has survived every one.

Creationists desperately try to deny the undeniable significance of the fossil record by various tactics, all logically dubious.  One tactic is to focus on a snap-shot of the fossil record as it exists today, ignoring the progress of discovery over time. At any moment in history there will be gaps in the fossil record. Creationists point out those gaps as if they call into question evolution. But when those gaps are filled, they just move on to other gaps or just point to the smaller gaps remaining.

They will also focus on the noise in the fossil record. A tiny percentage of living individuals will eventually be fossilized and discovered. Therefore the fossil record is a very low-res picture of the history of life. It is easy, therefore, to point to apparent discrepancies at below the resolution of the fossil record as if it calls into question the bigger picture that the fossil record can paint.

A proper scientific assessment of the fossil record, however, in terms of whether or not is supports evolutionary theory and common descent, is to look at how it is changing over time. What we see is that the more fossils we find the better the resulting picture matches predictions from evolutionary theory.  If evolution were not true, then we would predict the opposite.

As a side note – I am not talking about predicting what species evolved into what. The specific course of evolution is not something that is predictable, and evolutionary theory is not dependent upon any specific history of life. That is entirely a separate question.

A new study published by researchers at the University of Bath and London’s Natural History Museum provides more evidence for the success of evolutionary theory in predicting the pattern of the fossil record. They conducted a systematic survey of fossil dinosaurs. Evolutionary theory predicts that a morphological pattern and a temporal pattern of dinosaur fossils should overlap, and this is exactly what the researchers found. This adds statistical rigor to what was already apparent.

The researchers added that the tightness of overlap is dependent upon two things: the correctness of evolutionary theory, and the completeness of the fossil record. Therefore this study strongly suggests that not only is evolutionary theory correct, but at present we have a pretty complete picture of dinosaur evolution.

That’s cool. It’s also not surprising. Hunting dinosaur fossils has been a priority for paleontologists for over a century. And, dinosaur fossils are generally big. I assume this makes them easier to find.

It’s also a bit sad in that the evidence implies there are no major groups of dinosaurs in the mix waiting to be discovered. We have the big picture of dinosaur evolution pretty well sketched out. There are probably still many individual species waiting to be discovered, but not major groups. I suppose there can still be side branches that were regionally isolated, but if so then we do not have any individual specimens from these groups that do not fit into the bigger picture we already have. If there are any missing groups, they are completely missing.

When Darwin was alive and publishing his theory of evolution, we had only a smattering of dinosaur fossils. Now, 150 years later, we have a fairly complete picture of a temporal and morphological sequence of branching dinosaur evolution. Scientific theories are judged primarily on how well they make predictions about future discoveries. By that criterion Mr. Darwin’s theory is a resounding success.

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