The first vertebrates on Earth arose in shallow coastal waters

The cradle of vertebrate evolution was limited to a zone of shallow coastal waters, no more than 60 meters deep.

In those waters, fish — the first vertebrates — appeared roughly 480 million years ago, a study finds. For nearly 100 million years, those creatures rarely strayed from that habitat, where they diversified into a dizzying array of new forms, scientists report in the Oct. 26 Science. The study resolves a long-standing mystery about where our earliest backboned ancestors arose.

Scientists have long debated whether the animals appeared first in the shallows or the deep, or in fresh or salty water. “The main problem is that the fossil record [of vertebrates] is absolutely terrible for the first 50 million to 100 million years of their existence,” says paleobiologist Lauren Sallan of the University of Pennsylvania. “And when [there are] fossils, they’re in tiny pieces. It’s hard to tell what exactly’s going on.”

So Sallan and her colleagues amassed 2,827 fossils of jawed and jawless fishes that lived between 480 million and 360 million years ago. To that database, the team added information on the environments that the creatures lived in — such as shallow coastal water, freshwater or the deeper ocean — based on both the geology of the rocks the fossils were found in and the invertebrate fossils also found in the rocks.

Then, the researchers used mathematical calculations to predict the habitats of the most ancient vertebrates, filling in those gaps on the fish family tree. Rather than living in rivers or lakes or the deeper ocean waters around coral reefs, the first vertebrates stuck to a nearshore ocean environment, Sallan and colleagues found.

The creatures stayed in the shallows for about 100 million years, acquiring adaptations before eventually occupying different environmental niches, the researchers say. Some fish gained streamlined, graceful bodies good for fast swimming in deeper waters. Others had stronger, more armored bodies and stayed close to shore or moved into rivers or lakes.

Catalina Pimiento, a paleobiologist at Swansea University in Wales who wrote a commentary on the study in the same issue of Science, says the team’s results are “so robust.” And the find makes a lot of sense. “It’s just well-known that these coastal habitats [have supported] biodiversity,” she says, because such locations tend to offer both abundant food and shelter.

Ancient vertebrates’ diversification within those shallow waters may have been helped along by the rise of land plants during the Devonian Period, 419 million to 359 million years ago, and the subsequent increase in organic matter carried by rivers to the ocean (SN: 3/31/18, p. 9). That resulting bonanza of food, particularly floating food, may have helped to slowly give rise to more swimmers in addition to creatures already dwelling on the seafloor (SN Online: 7/17/18).

Other recent research has also identified shallow coastal waters as a cradle of diversity, finding that about 100 million years after the first vertebrates appeared, the earliest four-footed animals arose in salty estuaries (SN Online: 5/30/18.) The importance of such biodiversity hot spots makes it particularly worrisome that they also tend to be most threatened by human activities in modern times, Pimiento says. “These are our origins, and they’re the areas we’re degrading the most.”