Lighting the fuse for the Cambrian Explosion

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Pre-Cambrian microfossils may be early animals

Harvard paleontologists have shed new light on one of the most enduring mysteries of life on Earth: the origins of the creatures that suddenly appear in the fossil record 530 million years ago in an event known as the Cambrian Explosion.

In work that led to Australia and back, researchers believe that microfossils previously thought to be algae may actually be a specialized type of egg case, laid by an animal precursor for that explosion of life. The Cambrian Explosion was a watershed in the Earth’s biological history. Over a relatively brief span of geologic time, large-bodied, hard-shelled animals appear in the fossil record for the first time. The event is also known for the diversity of life it spawned, including almost all phyla of animals alive today. Phyla are broad classifications of life, such as Chordata (or vertebrates), which includes mammals, reptiles, and birds.

Scientists have long puzzled over the sudden appearance of these complex creatures because they must have evolved from precursors that appear to be missing from the pre-Cambrian fossil record.

Work led by Phoebe Cohen, a doctoral student in the lab of Andrew Knoll, Fisher Professor of Natural History and professor of Earth and planetary sciences, offers a reinterpretation of microfossils from the time just before the Cambrian, in the Ediacaran Period, which may solve the mystery. Published in the Proceedings of the National Academy of Sciences earlier this month, the work also included Knoll and Robin Kodner of the University of Washington.

“Phoebe’s work takes a previously enigmatic group of fossils and relates them directly to two major themes of Earth history: the emergence of animals and the expansion of oxygen-rich water masses in the oceans,” Knoll said.

Scientists have long known that pre-Cambrian rocks hold an abundance of unusual, microscopic fossils with hard shells, spines, hairs, and other protrusions. It has been thought that the fossils were merely different kinds of algae, known to be abundant in the pre-Cambrian seas and too different from animals to be the source of a sudden profusion of animal life.

“They were interpreted as an algal radiation, but they look like no known modern algae today,” Cohen said.

After examining hundreds of samples of these fossils, called acritarchs, and comparing them with both algae and the eggs of modern and fossil crustaceans — a group that includes shrimp — Cohen has come to a different conclusion. Rather than being algae, they most closely resemble a specialized egg created by modern crustaceans — called a resting stage — that is able to lie dormant for years waiting for favorable conditions before hatching.

If that is the case, the creatures that created those microfossil eggs would be tiny, but complex animal life, a potential precursor for the rapid diversification that followed.

The events occur at a time when the Earth’s atmosphere was growing richer in oxygen, which may have opened the door to larger-bodied creatures. Those first animals, Cohen said, may have predated the time of the Cambrian Explosion itself but be absent from the fossil record because their soft tissues weren’t preserved. Only later, when hard parts evolved, would they appear relatively suddenly as fossils.

Cohen had done some previous work with acritarchs, but had accepted the prevailing algae explanation until four years ago, when Knoll gave her a paper by marine biologists about recently discovered marine fossils of the resting eggs of small marine crustaceans called copepods. Intrigued, she began looking for more animal eggs to compare with the pre-Cambrian acritarchs.

“The more I found, the more I realized they look astonishingly similar to these Ediacaran fossils,” Cohen said.

After comparing their size, shape, and ornamentation, Cohen went further, examining their internal structure, finding that the pre-Cambrian microfossils more closely resembled the resting-stage egg of tiny animals than they do algae.

“Many invertebrate groups make resting stages similar to these Ediacaran fossils, and in a way that no other modern creature does,” Cohen said.