Major habitat shifts, such as moving to land from the sea, are key evolutionary events that often spark explosions of diversity. Until recently, it was unclear how frequent such habitat transitions have been across the eukaryotic tree of life, which includes animals, plants, and a vast array of microbes. Now, an international research team led by the Uppsala University in Sweden has discovered that eukaryotes have made hundreds of major leaps from sea to land and freshwater habitats, and vice versa, during their evolution over the last two billion years.
“We found that organisms in the eukaryote tree of life are generally grouped by whether they live in the oceans, or in non-marine habitats like freshwater and soils. This finding confirms that adapting to a different salinity – or crossing the salt barrier – is difficult, even for microbes,” said study lead author Mahwash Jamy, an expert in Organismal Biology at Uppsala.
“However, we found that microbial eukaryotes have still managed to successfully establish themselves in new habitats several hundred times during their evolution, and we’re likely to discover more of these transition events as we sequence microbes from more locations. It’s likely that these difficult-to-achieve transitions have allowed colonizing organisms to occupy vacant ecological niches, leading to the vast diversity of eukaryotes we see today.”
By sequencing DNA of microbes from boreal lakes, forest soils, the Indian Ocean, the Mariana Trench, and various other environments, the scientists reconstructed large evolutionary trees that revealed major patterns in the evolution of habitat preference. “It’s likely that two of the largest groups of eukaryotes, called SAR and Obozoa, each of which are much larger than the plant and animal kingdoms, arose in completely different habitats,” explained study senior author Fabien Burki, an associate professor of Organismal Biology at Uppsala.
“The SAR lineage, which includes groups like diatoms, ciliates, dinoflagellates, radiolarians etc., first arose in the Precambrian oceans. On the other hand, the ancestral Obazoan, which has now diversified into fungi, animals, choanoflagellates, and amoebozoans, likely inhabited non-marine habitats.”
These findings suggest that the salinity shift between marine and non-marine habitats played a crucial role in shaping eukaryotic evolution. Further research is needed to investigate what genetic mechanisms are underlying these seminal evolutionary events.
The study is published in the journal Nature Ecology & Evolution.