The first four-legged fins turned into paws during the development of land

American paleontologists from Harvard University found that the oldest tetrapods began to get out onto land and grow full limbs simultaneously. Previously, it was believed that first there was a transition, and only then changes in the humerus structure.

Changes in the shape of the humerus played an important role in adapting the ancestors of tetrapods to life on land. Accordingly, by studying this part of the skeleton’s evolution, we can predict when our ancestors left the ocean. It turned out that this happened simultaneously with the appearance of limbs.

Stephanie Pearce, professor at Harvard University in the USA

To track changes in the extinct quadruped structure, a group of researchers made 40 three-dimensional models of fossil humerus bones. Their representatives were just participants in this turning point in development.

The first true amphibians that could live on land almost permanently appeared about 20-30 million years after these fish left the ocean.

The humerus bone attaches the front leg to the body. It also contains many muscles and must withstand a lot of stress during limb movements. Therefore, the bone contains a large amount of important functional information related to the animal’s movement.

The researchers suggested that the evolutionary changes in the shape of the humerus, from short and squat in fish to more elongated and characteristic of tetrapods, had important consequences associated with the transition to a terrestrial lifestyle.

As a result of the analysis, the researchers found that the humerus of the first amphibians, such as Acanthostega or Ichthyostega, were already adapted for life on land, even though many paleontologists believed that these animals differed little in their way of life from their modern lobe-finned fish.

Soon, Pierce and her colleagues want to test these alternative theories. They plan to study how the shape of some other bones of the oldest amphibians and the last lobe-finned fish changed. For example, scientists want to check if the front and hind legs appeared at the same time.

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