Fundamental Science: The Discovery That Solved Darwin’s Dilemma

Darwin is known and celebrated for his theory of evolution, which caused a major paradigm shift in the way human beings view their role in the life of the planet. But perhaps not everyone is aware of one facet of it: the ability to point out the vulnerable aspects of the theory itself, which would be tested and resolved over the next century and up to the present day. One of these weaknesses is the famous “Darwin’s dilemma”.

The theory of evolution implies gradual changes and the emergence of new species through natural selection. But she had to face a major challenge: how to explain that at the time when Darwin proposed his theory, in the mid-19th century, there were no known fossils visible to the naked eye in sedimentary layers prior to about 510 million years, a period referred to as ” Cambrian explosion”?

In fact, fossils found in rocks deposited during the Cambrian explosion, such as the Burgess shales, in Canada, show a huge variety of complex animals, such as trilobites, marine arthropods then very widespread. Until the mid-20th century, almost a century after Darwin, fossils were still unknown in rocks older than these, deposited over the first 4 billion years of Earth’s history. Is the theory of evolution incorrect? Aren’t there animals older than trilobites, which suddenly “exploded” into many different forms during the Cambrian? This dilemma, the absence of fossils demonstrating the intermediate evolutionary phases until the arrival of the Cambrian explosion, was one of the main challenges to the theory of evolution.

Charles Doolittle Walcott himself, the American paleontologist who discovered the Burgess fossils in 1909, provided the first clues. Scouring all of North America, Walcott discovered microscopic fossils, invisible to the naked eye, in several Precambrian rocks, that is, deposited before the Cambrian period. Some of these fossils were later identified as ancient planktonic algae, and others were identified as possible bacterial fossils.

Some of these microorganisms even built colonies that formed stromatolites, structures recognizable in ancient rocks around the world. It was demonstrated that life had not emerged in the Cambrian suddenly and in its most diverse forms: there were much older remnants. These traces, however, are very different from what we are used to when we think of fossils, imagining a complete skeleton of a Tyrannosaurus Rex — they are more subtle and need to be read with the aid of equipment such as a microscope.

This situation, however, was to change quickly. In 1947, geologist Reginald Sprigg announced the discovery, in the Ediacar Hills region of southern Australia, of disc-shaped marks (possibly left by jellyfish-like creatures at the bottom of an ancient ocean). Sprigg believed he had found another deposit related to the Cambrian explosion, but a few years later paleontologist Martin Glaessner demonstrated that these sedimentary rocks were older – and the “Ediacaran fauna” became the oldest animal fossil pool.

Since then, fossils of the strange Ediacaran organisms have been seen in several other locations, and in 2004 a new geological period was formally defined, the Ediacaran, between 635 and 538 million years ago. The Ediacaran fauna ranges from the first organisms capable of building calcareous shells, to those with soft bodies that are difficult to fit into any subdivision of life.

Ediacaran fossils reveal not only the presence of complex animals before the Cambrian explosion, but also interesting ecological interactions between them. For example, in the limestone shells of some life forms you can see small holes, probably left by some predator that moved on the sea floor – this would have been the reason they developed a protective shell.

The discovery of Precambrian fossils and microscopic fossils in older rocks solves Darwin’s dilemma, but raises other questions, like all good scientific discoveries. Why do complex organisms not appear gradually in the fossil record, but rather clustered together in assemblages with different types of animal bodies at specific intervals in Earth’s evolution?

One line of thinking holds that the real problem is not the existence of bodies, but their preservation as fossils. The marks left by animals and the remains of their skeletons and shells undergo several processes after their death, with variations in several factors, such as temperature, pressure and chemical dissolution of the sediments until they become hardened and turn into sedimentary rocks. . In this process, the fossil record may disappear in part, or even completely. The Cambrian and Ediacaran fossil preservation sites would represent the moments in which a conjunction of favorable factors occurred, until their discovery, 500 million years later.

Another group maintains that the emergence and proliferation of various forms of life in a relatively short interval, that is, an “explosion”, is real and not a mere artifact of preservation. In this case, probably the genes that determine the characteristics of animals already existed long before and were only in a dormant state. Some external pressure, perhaps related to a sudden change in climate, environment, or the chemical composition of the atmosphere and seawater, may have prompted a rapid action of natural selection on these organisms, favoring those that developed characteristics that would allow them to survive. under the new conditions.

This kind of reasoning makes sense when we think that the first Ediacaran organisms emerged after the Earth passed through the Snowball, the biggest glacial event the planet has ever experienced. When the ice melted, the new environments may have acted promptly on potential genes, bringing about an accelerated diversification of species. A rapid diversification of traits, such as eyes capable of complex depth vision and the organization of bodies capable of light mobility, may have led to an arms race between predators and prey, in such a way that each adaptation adopted by one of the two ( for example, shells for protection) was readily answered by the other.

So we enter that spiral, which, 500 million years later, makes it possible for the readers of this text to use their mental apparatus, one of those most advanced evolutionary weapons, to look for traces of life in ancient sedimentary rocks and imagine what that life was like in ancient oceans. .

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Fabrício Caxito is a professor of geology, principal researcher in the GeoLife MOBILE project and philosopher at UFMG.

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