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Henry Gee, Rory Howlett, and Philip Campbell of Nature magazine have compiled a list of 15 scientific discoveries that have been included in Nature over the past decade or so that lend credence to the theory of evolution by natural selection. They call these “Evolutionary Gems.” This is number 1, which is included in the “Gems from the fossil record” category.
Land-living ancestors of whales
Fossils offer crucial clues for evolution, because they reveal the often remarkable forms of creatures long vanished from Earth. Some of them even document evolution in action, recording creatures moving from one environment to another.
Whales, for example, are beautifully adapted to life in water, and have been for millions of years. But, like us, they are mammals. They breathe air, and give birth to and suckle live young. Yet there is good evidence that mammals originally evolved on land. If that is so, then the ancestors of whales must have taken to the water at some point.
As it happens, we have numerous fossils from the first ten million years or so of whale evolution. These include several fossils of aquatic creatures such as Ambulocetus and Pakicetus, which have characteristics now seen only in whales — especially in their ear anatomy — but also have limbs like those of the land-living mammals from which they are clearly derived. Technically, these hybrid creatures were already whales. What was missing was the start of the story: the land-living creatures from which whales eventually evolved.
Work published in 2007 might have pinpointed that group. Called raoellids, these now-extinct creatures would have looked like very small dogs, but were more closely related to even-toed ungulates — the group that includes modern-day cows, sheep, deer, pigs and hippos. Molecular evidence had also suggested that whales and even-toed ungulates share a deep evolutionary connection.
The detailed study, by Hans Thewissen at Northeastern Ohio Universities Colleges of Medicine and Pharmacy in Rootstown and his colleagues, shows that one raoellid, Indohyus, is similar to whales, but unlike other eventoed ungulates in the structure of its ears and teeth, the thickness of its bones and the chemical composition of its teeth. These indicators suggest that this raccoon-sized creature spent much of its time in water. Typical raoellids, however, had a diet nothing like those of whales, suggesting that the spur to take to the water may have been dietary change.
This study demonstrates the existence of potential transition forms in the fossil record. Many other examples could have been highlighted, and there is every reason to think that many others await discovery, especially in groups that are well represented in the fossil record.
Reference
Thewissen, J. G. M., Cooper, L. N., Clementz, M. T., Bajpai, S. & Tiwari, B. N. Nature 450, 1190–1194 (2007).
Additional resources
Thewissen, J. G. M., Williams, E. M., Roe, L. J. & Hussain, S. T. Nature 413, 277–281 (2001).
de Muizon, C. Nature 413, 259–260 (2001).
Novacek, M. J. Nature 368, 807 (1994).
Zimmer, C. At The Water’s Edge (Touchstone, 1999).
Video of Thewissen’s research: www.nature.com/nature/videoarchive/ancientwhale
Author website
Hans Thewissen: www.neoucom.edu/DEPTS/ANAT/Thewissen
©2009 Macmillan Publishers Limited. All rights reserved.
Reprinted with permission per original article introduction.
Go To “15 Evolutionary Gems” #2
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