Those of us interested in science are being a bit spoilt at the moment – at least in the biological area. With the 150th Anniversary of the publication of The Origin of Species (24 November) and the 200th anniversary of Darwin’s birth (12 February) this year there are plenty of evolutionary science resources coming on line.
One example for the scientific journal are the 15 Evolutionary Gems from Nature. This downloadable publication includes gems from the fossil record, from habitats and from molecular processes. Another useful resource are the Year of Darwin series of videos from Case Western University.
Here I have combined these two resources with a short description of the discovery of intermediate fossils between fish and tetrapods as presented in the Nature publication and in Neil Shubins presentation in the Year of Darwin Lectures.
From water to land
The animals we are most familiar with are tetrapods – they are vertebrates (they have backbones) and they live on land. That includes humans, almost all domestic animals and most of the wild ones that any child would recognize: mammals, birds, amphibians and reptiles. The vast majority of vertebrates, however, are not tetrapods, but fish. There are more kinds of fish, in fact, than all the species of tetrapods combined. Indeed, through the lens of evolution, tetrapods are just one branch of the fish family tree, the members of which just happen to be adapted for life out of water.
The first transition from water to land took place more than 360 million years ago. It was one of the most demanding such moves ever made in the history of life. How did fins become legs? And how did the transitional creatures cope with the formidable demands of land life, from a desiccating environment to the crushing burden of gravity?
It used to be thought that the first landlubbers were stranded fish that evolved to spend more and more time ashore, returning to water to reproduce. Over the past 20 years, palaeontologists have uncovered fossils that have turned this idea upside down. The earliest tetrapods, such as Acanthostega from eastern Greenland around 365 million years ago, had fully formed legs, with toes, but retained internal gills that would soon have
dried out in any long stint in air. Fish evolved legs long before they came on land. The earliest tetrapods did most of their evolving in the more forgiving aquatic environment. Coming ashore seems to have been the very last stage.
Researchers suspect that the ancestors of tetrapods were creatures called elpistostegids. These very large, carnivorous, shallow-water fish would have looked and behaved much like alligators, or giant salamanders. They looked like tetrapods in many respects, except that they still had fins. Until recently, elpistostegids were known only from small fragments of fossils that were poorly preserved, so it has been hard to get a rounded picture of what they were like.
In the past couple of years, several discoveries from Ellesmere Island in the Nunavut region of northern Canada have changed all that. In 2006, Edward Daeschler and his colleagues described spectacularly wellpreserved fossils of an elpistostegid known as Tiktaalik that allow us to build up a good picture of an aquatic predator with distinct similarities to tetrapods – from its flexible neck, to its very limb-like fin structure.
The discovery and painstaking analysis of Tiktaalik illuminates the stage before tetrapods evolved, and shows how the fossil record throws up surprises, albeit ones that are entirely compatible with evolutionary thinking.
Daeschler, E. B., Shubin, N. H. & Jenkins, F A. Nature 440, 757–763 (2006).
Shubin, N. H., Daeschler, E. B., & Jenkins, F A. Nature 440, 764–771 (2006).
Ahlberg, P. E. & Clack, J. A. Nature 440, 747–749 (2006).
Clack, J. Gaining Ground (Indiana Univ. Press, 2002)
See also: Darwin’s Legacy 10 videos of lectures from Stanford University.
** Illustration by Kalliopi Monoyios