Researchers have uncovered a remarkable fossil site in a remote part of Canada's Northwest Territories, offering unprecedented insight into the earliest evolution of complex animal life on Earth. Findings from the site represent life from the Ediacaran biota—soft-bodied organisms that lived on the seafloor more than 500 million years ago—and push back the origins of animal movement and sexual reproduction by 5-10 million years. The work is published today in the journal Science Advances and led by researchers at the American Museum of Natural History and Dartmouth.
"For 3 billion years, life on Earth was dominated by microbes. Then, all the sudden, we get these strange-looking marine animals big enough to see and capable of behaviors we would find familiar today," said the study's lead author Scott Evans, assistant curator of invertebrate paleontology at the American Museum of Natural History. "If we want to understand this transition, when life first became large, complex and unmistakenly animal, this new site has tremendous potential."
With shapes that vary from flat discs to leafy fronds and ribbed ovals, Ediacaran fossils represent the first direct evidence of multicellular animal life. Ediacaran species are linked to a diverse set of animal groups, including mollusks, nematodes, comb jellies, and cnidarians (a group that spans jellyfish and corals). Others look nothing like any organism alive today but represent the oldest animals known to move in search of food or reproduce sexually.
Ediacaran life existed before most animals evolved the ability to form hard parts like shell or bone, so most are soft-bodied. Thus, these fossils are rare, requiring exceptional conditions to be preserved. While Ediacaran fossils have been identified on most continents except for Antarctica, only a handful of sites contain more than 10 different species, providing limited windows into this critical, roughly 40-million-year interval in Earth's history.
Scientists currently sort Ediacaran life into three groups, or "assemblages" representing different times in the geologic record: the Avalon assemblage (575-559 million years ago), the White Sea assemblage (559-550 million years ago), and the Nama assemblage (550-538 million years ago). Until now, fossils from the White Sea group had only been found in Europe, Asia, and Australia, not North America. In this study, the researchers found clear evidence of the White Sea assemblage in ancient rocks of Canada's Mackenzie Mountains on the traditional lands of the Sahtú Dene and Métis, who provided the research team with guidance and permission to access the site.
The discovery builds on earlier geological work in the region but represents a major step forward, with the researchers finding more than 100 fossils, including six groups never before seen in North America. But the biggest surprise was that some of these fossils are estimated to be about 567 million years old, 5-10 million years older than previously documented White Sea specimens and overlapping with the time of the older Avalon assemblage. Intriguingly, the layers in which fossils were found are overlain by hundreds of feet of potentially fossil-rich rock.
"Not only is this new site highly diverse, but also it is from a part of the rock succession where we have previously lacked fossil remains," said study co-author Justin Strauss, an associate professor of Earth and Planetary sciences from Dartmouth, who has been exploring this area for about 15 years. "This is really exciting. Given our understanding of the regional geology in northwestern Canada, there is great potential here to revisit our understanding of Ediacaran Earth history."
Among the finds made for the first time in North America are:
- Dickinsonia, a flat organism that moved around on the sea floor, lacking a mouth and instead absorbing bacteria and algae through its entire bottom surface, which Evans describes as a "bathmat" or "pancake" with a divided circular body
- an immobile tubular organism called Funisia that lived in clusters of similar size and offers the oldest evidence of sexual reproduction in the fossil record, likely with coordinated release of sperm and eggs into the water column like coral;
- Kimberella, with a muscular foot that fed by scraping the sea floor, widely interpreted as an early relative of mollusks and now potentially the oldest fossil bilaterian—the group of animals with distinct front, back, top and bottom with symmetric left and right sides that makes up more than 99% of all known animals
- and Eoandromeda, a possible comb jelly with eight spiral arms
The scientists also discovered that these organisms lived in deeper-water environments than previously recognized for the White Sea assemblage. This finding supports a growing hypothesis that early animals may have originated in offshore, deep-marine settings before expanding into shallower waters over time, the opposite of what is typical of animal evolution after this time.
"These results suggest a pattern where evolutionary innovation begins in deeper environments and later spreads toward the coast," Evans said. "We think of the deep ocean as a dark, inhospitable place, but it is also relatively stable, with few fluctuations in things like temperature and oxygen essential to most animal life. This stability may have provided key opportunities to support early animal life."
The fossils uncovered in this study will eventually be permanently accessioned by the Prince of Wales Northern Heritage Centre in Yellowknife, Northwest Territories.
Other coauthors on the study include Erik Sperling, Stanford University; and Kimberly Lau, The Pennsylvania State University.
This work was supported by a NASA Exobiology grant (# 80NSSC25K7024); a U.S. National Science Foundation (NSF) grant (# EAR-20 2143164); and NSF Frontier Research in Earth Science grants (# EAR-2021324 EAR-2021176).
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