Around 390 million years ago, vertebrates first moved out of the water and onto land. But how exactly this transition took place has not yet been scientifically clarified. Researchers have now analyzed 40 fossil humerus bones derived from fish, early terrestrial vertebrates, and transitional stages. Their results show that the first ancestors of the terrestrial vertebrates were still strongly attached to water. Adapting their limbs represented a compromise between both habitats.
The transition from water to land marks a turning point in vertebrate history. It is the starting point for the development of new ecosystems with a wide variety of terrestrial vertebrates (tetrapods). Previous research suggested that this evolutionary step began with creatures like the coelacanth that lived in water that kept drying out, creating evolutionary pressures to adapt to life on land. However, it has been known since the 1990s that the first tetrapods still had features such as gills and caudal fin. Her leg-like limbs are believed to have developed in the water. How and when they adapted to life on land is still unclear, especially since previous studies have been limited to a few fossil species.
Old and new fossils
Researchers led by Blake Dickson from Harvard University have now analyzed 40 fossil humerus bones from extinct animal species at the transition between water and land. They used fossils that had been known for a long time as well as those that were only recently discovered. According to the researchers, the upper arm bone (humerus) is particularly suitable for the investigations, as it occurs in all animal species relevant to the question and is often well preserved in the fossils. In addition, this bone is particularly relevant for the change from aquatic to land animals: depending on whether it is to enable flexible fin strokes or to withstand the weight of a land animal, it must be shaped differently.
“We expected that there would be a strong functional change in the humerus that we can use to predict at what point the tetrapods began to come ashore,” says Dickson’s colleague Stephanie Pierce. “We found out that terrestrial abilities seem to coincide with the origin of the limbs.” In fish, the bone is short and block-shaped, which is an optimal adaptation to swimming locomotion. In the case of land vertebrates, on the other hand, it is elongated, slightly twisted and strongly built in order to bear the weight of the animals and to enable walking. In fossils at the transition between water and land, the humerus has an L-shape – a kind of compromise solution.
Adaptive landscapes show adaptation
In order to show how well the different examined species were adapted to the respective habitats, the researchers used so-called adaptive landscapes. The perfect adaptation to a habitat is the summit of a mountain; forms that are less optimized for the habitat form valleys. An adaptive highlight for fish in this model is the optimal adaptation of the bone shape to swimming; for terrestrial vertebrates it is the optimal adaptation to locomotion on land. “We were able to use these adaptive landscapes to see whether the shape of the humerus in earlier tetrapods was better adapted to life in water or on land,” explains Pierce.
First, the researchers had to determine which properties of the humerus they wanted to include in the analysis. “It wasn’t an easy task because fish fins are very different from tetrapod limbs,” says Dickson. Ultimately, they settled on six characteristics, including the length and strength of the bone. The researchers now modeled on the computer how functional the respective variations were for locomotion in water and on land.
Opposite selection pressure
The result: the earliest tetrapods exhibited a unique combination of characteristics, but they did not correspond to an adaptive peak of their own. Instead, they were transitional forms that represented an only slightly optimized compromise between adaptations to water and land. In relation to the adaptive landscapes, they were thus in the valley at the foot of the mountain for terrestrial life. “That indicates there is potential for an increase in locomotion on land. But these animals had only developed a limited set of functional properties for effective land walking, ”said Pierce.
From an evolutionary point of view, the researchers explain this inadequate adaptation from today’s point of view by saying that the selection pressure for rapid adaptation to rural life was only low – after all, these animals were the first to colonize the new habitat, so that they do not participate on land, for example Predators had to expect. At the same time, there was selection pressure in the other direction at this point: the animals continued to look for food in the water, for example. Only when they freed themselves from these restrictions through changes in other areas did further optimization of the humerus make sense.
“Our study provides the first quantitative, high-resolution insight into the evolution of terrestrial locomotion at the transition from water to land,” says Dickson. In the future, the researchers want to carry out further investigations with their method of adaptive landscapes. “For example, there is the hypothesis that the front legs adapted to life on land in front of the rear legs. Our methodology could help to test this, ”says Pierce.
Source: Blake Dickson (Harvard University, Cambridge) et al., Nature, doi: 10.1038 / s41586-020-2974-5