The transition from water to land is a question that still intrigues scientists. Those ancient organisms would have needed to adapt to several new challenges to life out of water. So, how did they do it?
In a 2025 study, my colleagues and I tried to understand the genetic basis of adapting to life on land by comparing the genetic material of 150 living animals. We discovered that some adaptations to land are universal, while others are found only in a few lineages.
Animal life started in water over 600 million years ago. Around 500 million years ago animals began their journey from water to land. Known as the Cambrian period, this is one of the biggest evolutionary shifts in Earth’s history, that paved the way for all modern land-based ecosystems.
Although green plants transitioned to land just once around 500 million years ago, animals colonised land at multiple points in time independently. This makes animal life on land a striking example of “convergent evolution” – the process in which different lineages evolve solutions to the same problem. Each of these “jumps” onto land opened up new habitats and had a dramatic effect on the atmosphere and water cycle. This in turn created the modern ecosystems we live in.
Jialin Wei, CC BY-NC-ND
In our Nature paper, my colleagues and I explored these habitat transitions from a genetic perspective. First, we compared the genomes of more than 150 species across the animal kingdom to identify which genes are shared by different lineages. Then, using the evolutionary tree of animals, we mapped which branches of the tree those genes emerged or were lost in.
We found that most transitions to land were accompanied by a large gene turnover, with many gene gains and reductions happening at the same time. The ability of genomes to gain and lose genes played a key role in animal adaptation to new habitats.
Making the leap
This discovery led us to ask what these genes do and wonder why some were retained while others disappeared. Using analytical techniques and powerful computer tools, we found that genes repeatedly gained across distantly related landbased lineages were involved in functions related to dehydration. They were also often related to stress response (such as temperature, UV radiation, contaminants found on land, and toxic compounds from plants). The genes that were lost or diminished were often linked to regeneration, diet and biological clocks such as day and night cycles.
Life’s move from water to land profoundly reshaped the planet itself. As life ventured onto land, it changed Earth’s cycles, removing CO₂ from and increasing the amount of oxygen in the atmosphere. Land-based life also weathered rocks, which made them release more minerals like calcium into the ecosystem.
These findings suggest that genetic changes drove shifts in biological functions, which in turn became key drivers of the transition from water to land.
Some animals still need humid surroundings to thrive. For example, earthworms live in moist soil. In contrast, insects and mammals can live entirely on dry land. Interestingly, we found that semi-terrestrial species (mostly tiny invertebrates) tend to share more adaptations. For example, functions related to blood circulation and nutrient absorption that help them survive in soil.
Fully land-based animals seemed to evolve a wider diversity of adaptation strategies. We discovered gene innovations specific to certain lineages, such as genes for shell formation and mucus secretion in land snails and innate immunity genes in land vertebrates. Land-based animals evolved more reinforced and specialised barrier defences for life on land. These distinct traits reveal the unique evolutionary histories shaped by ecology, physiology and chance.
Our study also sheds light on when these transitions happens. We identified three major waves of water to land transitions over the past 500 million years, during the Ordovician (485–443 million years ago), Devonian–Carboniferous (419–298 million years ago) and Cretaceous periods (145–66 million years ago). These waves began with early land arthropods, such as insects, and ended with land snails like those found in our gardens.
These periods were probably triggered by dramatic ecological and geological shifts. For example the rise of early land plants and the creation of seasonal habitats that created new environments and opportunities for land-based animals.
Previous research has mostly focused on specific land animal lineages. However, our study brings these transitions together, offering the first comprehensive view of how and when animals conquered the land.
This study offers a glimpse into what might happen if we could replay the tape of life: some genetic changes seem inevitable, appearing again and again, as life adapts to land, while others are rare. Our research shows how evolution continuously finds new solutions to the challenges of life on Earth.
Jialin Wei is supported by University of Bristol-China Scholarship Council joint-funded Scholarship.
