“Missing” links?

“Missing link” is an inappropriate term when referring to a transitional form not yet discovered in the fossil record. “Missing” usually describes something that was supposed to be present but has disappeared for unknown reasons — if, for example, a favorite book suddenly disappeared from your room But you wouldn’t describe the book as “missing” if you had lent it to a friend and expected it to be gone. The same arguments apply to so-called “missing links.” Biologists know that fossilization is a chancy process — most things that have lived on Earth are eaten or rot away before they can be fossilized. Very few organisms wind up in situations in which fossilization is possible and have body parts that can be preserved in fossils. Thus, biologists expect that most intervening steps of an evolutionary transition (such as vertebrates’ invasion of the land) will not be preserved in the fossil record. Occasionally, we get lucky and discover a transitional form that has been preserved in a fossil — but that does not imply that its ancestors, descendants, or other organisms that once lived on Earth are “missing.”

News update, June 2009

Since their initial publication on Tiktaalik, paleontologists Neil Shubin, Edward Daeschler, and Farish Jenkins, have continued to study its remarkable fossils. This past year, the team, now joined by Jason Downs, announced the results of a detailed analysis of Tiktaalik‘s skull.

Like its limbs, Tiktaalik‘s skull has a mix of traits that resemble fish in some ways and land-dwelling tetrapods in others. For an example, just look to your right or left. We tetrapods have necks, and so can keep our bodies still while moving our heads. Modern fish and Tiktaalik‘s ancestors, on the other hand, are neck-less and cannot. In this respect, Tiktaalik was more like us. Tiktaalik had lost some bones in its head, giving it a more mobile neck — which would have allowed it to rest its body on the bottom of a shallow pool and still turn its head to snap up food.

The new study also revealed that the bones of Tiktaalik‘s skull were somewhere between tetrapods’ and fishes’ in terms of how much they could move relative to one another. Our skulls, for example, are rigid and fused, whereas Tiktaalik‘s ancestors had more joints in their skulls, which allowed the bones to move to help with feeding and breathing underwater. Tiktaalik‘s skull fell between these two extremes. It still had some fish-like joints, but had evolved to be more rigid and stiff than that of its ancestors.

A cranial bone called the hyomandibula also illustrates how Tiktaalik helps connect ancient fish to modern tetrapods. In fish, this bone is large, attaches the upper jaw to the braincase, and helps with gill-breathing. In land-dwelling tetrapods, this bone is tiny — and in fact, has evolved an entirely new function: helping us hear. It became the stapes, one of the bones in your middle ear! The new research revealed that Tiktaalik‘s hyomandibula is smaller than that of its ancestors, but not nearly as small as the stapes. This probably means that Tiktaalik had evolved to rely less on pumping water across its gills to get oxygen — and that, in Tiktaalik‘s time, the hyomandibula had already begun the evolutionary journey that would eventually reshape it into the stapes.

Though these new findings are interesting, they are not surprising. Based on many other lines of evidence, we already knew that modern tetrapods evolved from a fish ancestor. Tiktaalik‘s cranium represents the mix of fish-like and tetrapod-like characteristics that we would expect to find in an organism closely related to the ancestral tetrapod. The new research simply reveals with even more clarity exactly how and when this gradual evolution occurred.

News update, June 2010

In 2010, scientists announced the discovery of fossil footprints that may call into question the timing of the evolution of four-legged vertebrates (i.e., tetrapods). Tiktaalik represents a close relative of the ancestor of tetrapods, and its fossils date to 375 million years ago. The first unambiguous fossils of tetrapod bones (e.g. Acanthostega) date to just after that time. These and many other lines of evidence support the idea that the first tetrapods evolved around then, probably between 385 and 391 million years ago. However, that date now seems less certain.

In January of this year, a group of Polish and Swedish paleontologists announced the discovery of nearly 400 million-year-old fossil footprints that seem to have belonged to a fully-formed tetrapod. If a full blown tetrapod was around 400 million years ago, the earliest tetrapods must have evolved long before then! Tiktaalik‘s fossils may be younger than the first tetrapods.

If scientists come to a consensus about the date of these footprints and the fact that they belonged to a tetrapod, would it contradict Tiktaalik‘s status as a transitional form? No. Tiktaalik‘s position on the evolutionary tree of tetrapod ancestors wouldn’t change a bit — after all, Tiktaalik would still have all the characteristics that help us understand the order and way in which tetrapods evolved. However, it would imply that Tiktaalik and early tetrapods like Acanthostega have long ghost lineages — a series of ancestors that lived but did not leave behind a fossil record. In other words, our best hypothesis regarding the evolutionary relationships among these organisms would not change, but we’d have to reevaluate the likely dates we assign to different branching points on the family tree of tetrapods and move many of them back in time.