Science: 567-million-year-old fossils change perspective on evolution

A discovery in the Mackenzie Mountains of Canada's Northwest Territories is pushing back some of the most important milestones in animal evolution by up to 10 million years. Researchers have identified more than 100 fossils from the Ediacaran period, about 567 million years old, including organisms that moved along the seafloor and clues to sexual reproduction. The study, published in the journal Science Advances, suggests that complex animal life was already more diverse and sophisticated than previously thought before the Cambrian explosion.

• A window into the world before the Cambrian explosion

The fossils come from an extremely isolated area in the Mackenzie Mountains of Canada's Northwest Territories, which was covered by ocean about 567 million years ago. Researchers have identified more than 100 fossils here belonging to the Ediacaran biota, a geological period before the Cambrian, when some of the first large multicellular life forms appeared in Earth's oceans. The discovery is important not only for the age of the fossils, but also for their diversity. The site includes six groups of organisms that had never been identified before in North America. In addition, the fossils come from a deeper marine environment than where Ediacaran organisms are usually found. For paleontologists, the information changes the picture of a period when complex life was just beginning to take recognizable forms.

• Animals that moved millions of years earlier

One of the most important implications of the discovery is related to the emergence of animal mobility. Among the identified fossils is Dickinsonia, a flat, segmented organism that researchers have likened to a "pancake" or a mat, reports amnh.org (American Museum of Natural History). Fossil analysis indicates that these organisms were able to move along the ocean floor in search of food.

The discovery thus pushes back, by millions of years, the moment when complex organisms acquired the ability to move actively. Until now, researchers believed that some more complex forms of mobility became widespread later in the Ediacaran. However, the Canadian fossils show that mobile organisms already existed about 567 million years ago. This is an important difference. The ability to move fundamentally changes the way an organism interacts with its environment: it can search for food, avoid unfavorable conditions and occupy new areas of the ecosystem.

Another discovery is related to reproduction. Among the organisms identified is Funisia, a tubular form, similar to marine colonial structures. Researchers believe that this represents one of the oldest known evidence for sexual reproduction. The hypothesis is that the organisms released reproductive cells into the water in a coordinated manner, in a mechanism broadly comparable to the reproduction of modern marine organisms such as corals. If the interpretation is correct, the emergence of this mechanism can be pushed back by about 5-10 million years. For the evolution of life, sexual reproduction is a fundamental moment because the combination of genetic material from individuals accelerates the genetic diversity of populations.

In the case of Ediacaran organisms, however, researchers have to work with extremely old fossil evidence, and interpreting the behavior of extinct organisms is inevitably more difficult than identifying anatomical structures.

• A world dominated by organisms that no longer exist

The Ediacaran biota is one of the most unusual periods in the history of life. The organisms discovered in these rocks do not always resemble contemporary animals. Some were leaf-shaped or feather-like, others were flat and segmented, and some had structures that have no obvious equivalent in modern fauna, according to cnas.ucr.edu (College of Natural &Agricultural Sciences). However, these organisms demonstrate that multicellular life had already become capable of reaching considerable sizes and developing complex behaviors. Research on Ediacaran biota has previously provided evidence for the emergence of mobility, active feeding and sexual reproduction before the Cambrian explosion. The new Canadian site, however, extends the range in which these characteristics can be observed in the fossil record.

• Complex life may have begun in the depths

One of the most interesting conclusions of the study relates not only to the age of the organisms, but to where they lived. The researchers believe that the area where the fossils were found represented, 567 million years ago, a deep-sea environment. The rocks do not show the characteristic traces of turbulent waters, which supports the hypothesis of an environment further from the coast. This conclusion may modify an older hypothesis about the emergence of complex animal life. One traditional explanation is that the evolution of large, complex organisms was initially favored by shallow marine environments, where there were sufficient resources and where light penetrated the water. The new data suggest the opposite scenario.

• The deep ocean, an evolutionary laboratory

According to the researchers, deeper marine environments could have offered an important advantage: stability. Near the coast, the temperature, oxygen levels and physical conditions of the water can vary more. In the depths of the ocean, changes are generally slower. This stability could have created favorable conditions for the development of complex organisms. The "deep sea” should therefore not be viewed exclusively as a dark and hostile environment. At certain times in Earth's history, its relative stability could have provided a favorable environment for evolutionary innovation. The researchers therefore propose a model in which certain biological innovations emerged in deeper marine environments and later spread to coastal areas.

• Kimberella, a possible link to modern animals

Among the fossils discovered is Kimberella, an organism of particular interest to paleontologists. It had a more complex body structure and a possible muscular "foot,” which allowed it to move and feed by scraping the surface of the seabed. Kimberella is considered a possible early representative of the bilaterian group, the category that today includes over 99% of known animals. Bilateria have a body structure characterized, broadly speaking, by the existence of a front and a back part, as well as by bilateral symmetry. If the interpretation is correct, the fossils provide information about an extremely early stage in the evolution of the line that would eventually lead to much of today's fauna.

• How the discovery changes the evolutionary timeline

The importance of the research is not that scientists have identified the "first animals” on Earth. The origins of animals are much older and remain a hotly debated topic. Instead, the Canadian fossils provide information about when multicellular organisms began to show features that we associate with complex animals today. The new research shows that some of these features were already there about 567 million years ago, which is earlier than some interpretations of the fossil record suggested. Instead of an evolution in which simple, sedentary organisms were suddenly replaced by more complex forms, the Canadian site suggests that several types of organisms coexisted for millions of years. This picture is more of a gradual and experimental evolution.

The discovery also provides insight into the scale of geological time. Dinosaurs would not appear until hundreds of millions of years later. The first complex land plants, forests, and land animals belong to much more recent eras. By 567 million years ago, however, Earth's oceans were already home to organisms large enough to leave visible traces in rock. For about three billion years, life on Earth had been dominated by microscopic organisms, according to the American Museum of Natural History. The emergence of large, complex organisms was therefore one of the most important transitions in the history of the biosphere.

The site in the Mackenzie Mountains is particularly valuable because it preserves a diverse fossil community in a single area. Researchers have been working in the region for many years, and access to the site is difficult. Expeditions require long drives and, at times, helicopter transport. The isolation of the area partly explains why these formations have been so little studied until now. The study was published in the journal Science Advances and was carried out by a team led by paleontologists from the American Museum of Natural History, in collaboration with researchers from Canada and the United States.