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Image: Development of forelimbs
Science / Illustration by Emese Kazár
This chart shows differences in a variety of creatures that lived roughly 360 million to 410 million years ago. The sequence is arranged in a way that shows how fins might have evolved into limbs.
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Science
updated 4/1/2004 2:35:16 PM ET 2004-04-01T19:35:16

There is something fishy going on in your arms and legs — and it’s a good thing. With the discovery of the world’s oldest known arm bone, scientists conclude that many of the physical features we associate with life on land, including the bone structures and muscles necessary for walking and doing pushups, have their evolutionary roots in fish.

Analysis of this newly discovered fossil arm bone, in addition to other recent finds, overturns the long-held idea that limbs evolved specifically for life on land. A full description of the 365-million-year-old arm bone from Pennsylvania appears in Friday's issue of the journal Science, published by AAAS, the nonprofit science society.

“We are trying to untangle a very complicated and important evolutionary event — the shift from water to land,” said study author Neil Shubin from the University of Chicago.

The upper arm bone, or humerus, that was discovered by the authors belongs to a new variety of four-limbed carnivore that lived long before the dinosaurs. These carnivores were some of the first tetrapods, members of a group that includes all creatures with limbs, including reptiles, amphibians, birds and mammals.

“This is the earliest known arm bone. It’s a mosaic of fish features, tetrapod features and unique features,” Shubin explained.

Image: Cover of Science
Art by Kalliopi Monoyios
The cover illustration for the journal Science reconstructs the posture of an extinct animal from isolated bones. The humerus indicates that the creature could support the front end of its body in a low, wide stance. This posture reflects an intermediate stage in the evolution of amphibian limbs from fish fins.
Shubin and his colleagues compared the arm fossil to the bones of both ancient and modern fish, and arrived at a new understanding of the beginnings of limb evolution.

“The hybrid creature used its powerful limbs to do some of the world’s first pushups,” Shubin said.

The new fossil, like a photo taken with high-speed film, catches limb evolution in action. It helps clarify when, where and how animals first learned to walk. The fossil evidence suggests that the first limbed animals tried all kinds of walking strategies. Without Devonian photo albums full of fish trying to put weight on their pectoral fins, paleontologists must rely on the fossil record to understand how new fossil finds fit into the story of life.

“This humerus is quite unique,” said Jennifer Clack, the author of an accompanying article that provides some background information on this fossil.

Clack said the fossil’s “bizarre shape” does not fit the patterns of other early tetrapods.She believes that there were a variety of limb shapes, sizes and strengths among early tetrapods experimenting with adaptations for life on land.

Plants help limbs grow
Before vertebrates took their first steps — in or out of the water — plants with stems, roots and leaves colonized shallow rivers and changed the nature of many aquatic environments and nearby shores. Plants grew thick in streams and rivers, their roots stabilized ecosystems, and their decomposing biomass generated organic muck. Plant-clogged waterways made weight-bearing fins, and eventually limbs, useful for getting around, Shubin suggests.

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The fossil also shows that tetrapod shoulders changed the direction they face in order to adapt to life in a plant-tangled, aquatic environment. With shoulders facing sideways, the tetrapod’s arms projected out from its trunk at a right angle, similar to the limb arrangement of a crocodile. This shoulder and arm orientation would have made the front limbs more useful as weight-bearing limbs than paddles.

In addition to altering land and water habitats in ways that probably encouraged limb development, the rise of more advanced plants also changed the oxygen balance. Plants contributed to dropping oxygen levels in water and rising oxygen concentrations in the air. This oxygen scenario, combined with the newfound ability to stick their heads out of the water, may have encouraged the evolution of lungs in fish.

Streams full of toothy carnivorous fish — some with joints in their skulls to increase the size of their bites — may have also inspired tetrapods to take their head-propping, muck-stomping skills to higher ground.

Fish are family, too
The evolution of limbs from fins, one of the key transitions in the history of life, took place gradually and largely separate from the water-to-land transition.

After comparing this fossil arm bone to those from other tetrapods and closely related fish, the scientists conclude that many of the limb and muscle developments needed for walking on land developed before fish and limbed animals split.

For example, some modern fish can hold the same spot or lift up their trunks in moving water, thanks to bone and muscle changes that occurred before fish and limbed animals went their separate ways.

Honk if you love fossils
The scientists discovered the fossil in 1993, but they only recently exposed the entire bone from its red sandstone encasement and realized the significance of the specimen. The discovery site, Red Hill, holds a series of ancient stream deposits that preserve the birth, life and death of tropical to subtropical ecosystems from the Late Devonian period of Earth’s history. Red Hill includes a wall of rock along the shoulder of a highway in Pennsylvania’s Catskill Mountains.

If the scientists discovered this arm bone along a shoulder of a highway, perhaps the secrets to tetrapod ankles and wrists are resting underneath the foot of a nearby mountain.

© 2013 American Association for the Advancement of Science

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