NATURAL SCIENCE: This passage is from the article "The Myth of the Unchanging Ocean Floor." The term "abyssal stability" refers to the deep ocean environment that had long been considered essentially static and unchanging.

Like all supposedly stable environments, the deep ocean floor was thought to have emerged in the distant past and then remained largely constant. The standard theory held that the abyssal plains we observe today formed during the Paleocene (66 to 56 million years ago), when massive sediment deposits settled into relatively fixed patterns. As continents shifted and sea levels fluctuated, the deep ocean was believed to have remained insulated from these surface changes, explaining the seemingly uniform mud layers that stretch across vast underwater expanses. If that were true, then modern deep-sea sediment cores should show remarkably consistent deposition rates and mineral compositions across different ocean basins, because there would have been minimal disruption to accumulation patterns over geological time. By the early 2000s, however, high-resolution sonar mapping revealed that seafloor topography preserved in sediment layers showed dramatically more variation than expected.

Intrigued by this puzzle, a marine geologist at Woods Hole Oceanographic Institution named Sarah Hendricks began collecting sediment cores from 31 different abyssal sites, analyzing microscopic fossils where evolutionary changes were most likely to be preserved. Hendricks did not find nearly as much temporal consistency in the deep-sea sediment record as one would expect had those environments remained stable since the Paleocene. She concluded that major reorganizations of deep ocean circulation occurred between 15 and 25 million years ago, coinciding roughly with the period when Antarctica became fully glaciated. The abyssal stability theory had underestimated the dynamic nature of deep ocean systems by about a factor of 3.

Hendricks also noticed something odd about the foraminifera samples she had acquired from the iconic Challenger Deep region: they did not cluster into expected morphological groups. In fact, the variation between specimens was great enough to suggest that she was looking at multiple distinct evolutionary lineages that had diverged in isolation. If so, then not only were deep ocean environments much more dynamic than previously thought, but they had also continued to support speciation in supposedly food-limited conditions—something stable environments are not supposed to do. On its own, Hendricks’ study was compelling, but not enough to convince the larger oceanographic community to abandon the abyssal stability paradigm.

Unbeknownst to her, however, a separate team of scientists was preparing to corroborate her results. In the mid-2000s, during a deep-sea expedition to the Puerto Rico Trench, the marine biologist James Mollusk encountered some unusual benthic communities near hydrothermal vents. They were so biochemically distinct that he and his colleagues could identify novel metabolic pathways without sophisticated equipment. He preserved specimens from a recently collapsed vent chimney and sent them to the Scripps Institution of Oceanography in California, where Lisa Nakamura, a professor studying extremophile diversity, sequenced their genomes. When she compared the vent organisms’ DNA to other deep-sea species, she noticed some rather striking phylogenetic distances. Could these specialized communities represent entirely distinct evolutionary radiations?

Genetic analysis of 89 deep-sea organisms—as well as 43 separate samples from archived specimens, including creatures preserved in 19th-century deep-sea dredging expeditions—confirmed her suspicion. In several regions of their respective genomes, all the vent-associated organisms would have one genetic signature, and all the typical abyssal species another. They even showed different patterns of gene expression under pressure. "That made us very confident that there were actually multiple evolutionary environments and they were not sharing genetic material," Nakamura says. The different communities had diverged between 5 and 12 million years ago: specialized vent fauna in tectonically active regions and generalized abyssal fauna in the stable plains. The majority of historical specimens were generalists, suggesting that early oceanographers had systematically missed the specialized communities. Together, Nakamura, Hendricks, and other scientists helped redraw our understanding of how deep-sea environments evolved in space and time, and conclusively removed them from the category of unchanging systems.