It has been a hundred and twenty-five years since Charles Darwin passed away, but his legacy is alive and well. The theory of evolution which he was first to propose has become the unifying pillar of modern biology, drawing together a vast array of evidence from genetics, paleontology, biochemistry, ecology and other scientific fields. Contrary to the creationists who are perpetually (and wrongly) forecasting its imminent demise, today evolution is in better shape than it has ever been. And one of the strongest lines of evidence supporting it comes from the fossil record. In sharp contrast to the deceitful arguments of creationists, the painstaking study of Earth’s rock record has yielded a wonderful wealth of fossils showing evolutionary change in progress: “more than enough,” as the famous evolutionary scientist Stephen Jay Gould wrote, “to convince any fair-minded skeptic about the reality of life’s physical genealogy.”
Two exciting new papers from the journal Nature offer further support for this. Say hello to Tiktaalik roseae:
This astonishing creature is one of the most important finds so far in the tetrapod series – the transitional fossils that detail the evolution of one lineage of fish into four-legged creatures that walked on the land. Tiktaalik hails from the Late Devonian period, about 360 million years ago, and is both chronologically and morphologically intermediate between two other major fossils in this series, the more fish-like Panderichthys and the more tetrapod-like Acanthostega.
Tiktaalik‘s skeleton was discovered in the Canadian Arctic, but it did not live on the icy tundra. The world was a very different place in the Late Devonian: continental drift had sutured together the ancient landmasses of Laurentia, which incorporated Greenland and North America, and Baltica, which incorporated most of modern Europe. The collision created the new continent of Euramerica and triggered a wave of mountain-building all along America’s eastern seaboard, creating the range known today as the Appalachian Mountains. Although the Appalachians are ancient and low today, worn down by hundreds of millions of years of erosion, the violence of the continental collision may have made them as tall as the Himalayas when they were first created.
In those days, the Canadian Arctic lay near the equator. Today it is a frigid tundra, but in the Late Devonian it was a tropical swamp, drained by a web of lakes, rivers and streams. Giant ferns, horsetail plants and club mosses grew along the water’s edge, while forests dominated further inland, made up of primitive trees such as Archaeopteris that grew up to sixty feet high and reproduced via spores. By this time the invertebrates had already moved onto the land – insects, spiders, scorpions – but no vertebrate animals had. Were we given the chance to observe it, it would be a world of wild and unearthly beauty: dense, marshy forests with strange plants in verdant profusion, rivers choked with algae, fungi growing on fallen logs and dropped leaves. But there would have been no birdcalls, no croaking frogs: aside from the wind and the waves and the buzzing of insects, it might well have been a silent jungle.
This was the world of Tiktaalik and the early tetrapods. It was once believed that legs first evolved in barren, drought-wracked plains, where primitive amphibian-like fish could use their newly adapted limbs to drag themselves to a new pond when their home waters dried up. But the swampy habitat of the Late Devonian disproved that idea. The current thinking is that legs first evolved not for crawling out of the water, but for moving around while still in the water. Other early tetrapods such as Acanthostega had unmistakable limbs, but due to the arrangement of the joints, their legs could not have supported the animal’s weight on dry land. They may have been used, instead, for sculling along the bottom in turbid, silty lagoons, or so the animal could raise its head out of water to breathe when algae blooms choked all the oxygen out of rivers and streams (lungs, like legs, evolved before vertebrates left the water).
Tiktaalik fits this pattern to a T. In most respects its body is fish-like: it has fins and gill arches, just like a fish. However, its skull and especially its limbs mark it as a tetrapod ancestor. Species such as Panderichthys had true fins, similar to those of modern ray-finned fishes, consisting of an array of long, thin, spindly bones unsuitable for bearing weight. On the other side of the gap is Acanthostega, with true limbs – each containing a radius and an ulna, just like our arms, and outfitted with eight true toes. Tiktaalik is intermediate between the two, with limb-like fins terminating in stubby bones that were neither fin rays nor true digits. In addition, the skull of Tiktaalik is very tetrapod-like, and notably, it has a neck. This is a tetrapod feature not found in fishes, whose skull and shoulder girdle are joined together. Tiktaalik‘s shoulder girdle is stout and strong, capable of bearing weight, more like a tetrapod than a fish. Its ears, too, are an intermediate characteristic. In the course of evolution, the hyomandibular bone, which in fish helps to articulate the jaw, in mammals shrank, migrated deeper into the skull and became the stapes, the “stirrup” bone used to transmit sound vibrations into the inner ear. Tiktaalik‘s hyomandibular bone shows this transition in progress: it has shortened like the mammalian stapes, but has not yet moved back into the skull.
Not only is this find a compelling example of evolutionary transition, it is an example of how the theory of evolution can be used to make accurate predictions about the physical world. The scientists who discovered Tiktaalik made a threefold prediction, based on evolutionary theory: that such a creature would exist, that it would be found in rocks in a certain location, and that it would be found in rocks of a certain age. They went to this area explicitly because other primitive tetrapods had been found there, and searched in Late Devonian strata because more fish-like creatures were known from earlier strata and more tetrapod-like creatures were known from later strata. And all three of these predictions were borne out by the evidence.
For more, see Martin Brazeau’s The Lancelet, Carl Zimmer’s The Loom and P.Z. Myers’ Pharyngula, or from the New York Times, Scientists Call Fish Fossil the ‘Missing Link’. The citations for the two original Nature papers, along with a third article providing additional commentary, are as follows:
Edward B. Daeschler, Neil H. Shubin and Farish A. Jenkins, Jr. “A Devonian tetrapod-like fish and the evolution of the tetrapod body plan.” Nature 440, p. 757-763 (6 April 2006).
Neil H. Shubin, Edward B. Daeschler and Farish A. Jenkins, Jr. “The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb.” Nature 440, p. 764-771 (6 April 2006).
Per Erik Ahlberg and Jennifer A. Clack. “A firm step from water to land.” Nature 440, p. 747-749 (6 April 2006).
Image credit: Eduard Solà, released under CC BY-SA 3.0 license
