Land animals evolved from marine ancestors – a new study reveals the genes of the transition

The transition from water to land is a question that still puzzles scientists. Those ancient organisms would need to adapt to the many new challenges of life outside of water. So, how did they do that?

In the 2025 study, my colleagues and I tried to understand the genetic basis of adaptation to life on earth by comparing the genomes of 150 living animals. We found that some adaptations are universal, while others are only found in a few generations.

Animal life began in water more than 600 million years ago. About 500 million years ago animals began their journey from water to land. Known as the Cambrian period, this is one of the biggest changes in Earth’s history, which paved the way for the modern land-based ecosystem.

Although green plants evolved to settle only once in the last 500 million years, animals have independently colonized the land for many years. This makes animal life in the wild a fascinating example of “evolutionary evolution” – the process by which different generations develop solutions to the same problem. Each of these “jumps” in land opened up new habitats and had a dramatic effect on the air and water cycle. This in turn created the modern environment in which we live.

In our Nature paper, my colleagues and I analyzed these habitat changes from a genetic perspective. First, we compared the genomes of more than 150 species across animals to determine which genes are shared across lineages. Then, using the animal evolutionary tree, we mapped out which branches of the tree appeared or were lost.

We found that most changes to the landscape are accompanied by significant genetic change, with many genetic gains and losses occurring simultaneously. The ability of genes to gain and lose genes has played an important role in the adaptation of animals to new environments.

Jumping

This discovery led us to ask what these genes do and why some were preserved while others disappeared. Using analytical methods and powerful computer tools, we found that genes that were frequently found across streams related to geographic areas were involved in activities related to dehydration. They were often associated with stress responses (such as heat, UV rays, pollutants found in the soil, and toxic compounds from plants). The genes that were lost or reduced were often associated with regeneration, diet and biological clocks such as the day and night cycle.

The migration of life from water to land profoundly changed the planet itself. As life descended, it changed Earth’s cycles, removing CO₂ and increasing the amount of oxygen in the atmosphere. The soil organisms also collided with the rocks, which caused them to release large amounts of minerals such as calcium into the environment.

These findings suggest that genetic changes led to changes in the functioning of organisms, which became the main motivation for moving from water to land.

Some animals still need a moist environment to thrive. For example, earthworms live in moist soil. On the other hand, insects and mammals can survive completely on dry land. Interestingly, we found that semi-terrestrial species (especially small invertebrates) tend to share many levels. For example, functions related to blood flow and absorption of nutrients that help them survive in the soil.

Animals living entirely on land appeared to develop a wide variety of adaptations. We discovered lineage-specific genetic modifications, such as genes for shell formation and mucus secretion in land snails and innate immunity genes in vertebrates. Land-based animals became reinforced and specialized barriers to life on earth. These unique traits reveal a unique evolutionary history shaped by ecology, physiology and chance.

Our study also sheds light on when these changes occur. We have identified three major tides to change the world in the last 500 million years, the Ordovician (485–443 million years ago), Devonian–Carboniferous (419–298 million years ago) and Cretaceous (145–66 million years ago) periods. These waves began with the first land arthropods, such as insects, and ended with land snails such as those found in our gardens.

These periods were probably caused by major environmental and geological changes. For example the rise of the first land plants and the establishment of spring habitats that created new environments and opportunities for land-based animals.

Previous studies have focused on a particular lineage of land animals. However, our study ties these changes together, providing the first comprehensive view of how and when animals conquered the land.

This study sheds light on what might happen if we replayed the tape of life: some genetic changes seem inevitable, they occur frequently, as life adapts to the landscape, while others do not. Our research shows how evolution continues to find new solutions to the problems of life on Earth.