The very first vertebrates came from the beach.

Science News looks back at the shallow coastal waters of eons past, when the wading was comfortably uncrowded and the first creatures with spinal cords began to take shape:

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.

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. 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, [Swansea University paleobiologist Catalina] Pimiento says.