Branching off the Tree of Life

Overshadowed by the remains of dinosaurs and hominids, the Cambrian period fossils tell the story of a revolution in evolution that sheds light into our ultimate origins.

Burgess Shale does not look like an eventful place. Located in the Canadian Rocky Mountains, this old quarry appears to be desolate place, devoid of any peculiarities and far away from human civilization. The whole picture changes when one looks at the rocks that give this site its name. They show the signs of an evolutionary revolution that changed the course of the history of life. Because Burgess Shale is, in other words, the flagship paleontological site of the Cambrian explosion, the phenomenon that gave rise to most modern animals.

Hard bodies for a hard world

The Cambrian period, the first division of the Palaeozoic era, began 541 million years ago. Life had existed on Earth for several thousand million years already, but the fossils of the organisms that crawled in the primigenial oceans that covered most of the surface of our planet show that those creatures were mostly unicellular. Sometimes they formed colonies of individuals that represented the contemporary pinnacle of evolution, even simpler than a modern sponge. Then, in a period of 40 million years (which is a heartbeat in terms of geological time), the oceans saw the diversification of most animal phyla.

Phyla are the immediate level after kingdoms in cladistics. We are members of the phylum Chordata, those animals who possess a spinal cord; bees and crabs are Arthropoda, having six legs and an exoskeleton, and so on. There are over 30 animal phyla, and, while it is suspected that their origin is pre-Cambrian, it was only in this period when the diversity of multicellular species occurred in all ecological niches.

The causes of this explosion are not yet fully understood. Palaeontologists have suggested that the start of the Cambrian was marked by an increase of oxygen levels in the atmosphere, which allows organisms to grow larger, as the limitations of the square-cubic law that regulates oxygen need in eukaryotes (related to the surface and volume of creatures) dampen if the overall concentration of the gas increases. Other physical factors include the formation of the ozone layer, which protects Earth’s life from space radiation, and the abundance of calcium due to volcanic activity, which allowed the formation of hard exoskeletons.

However, evolutionary causes must be taken into account. The previously mentioned ability to produce exoskeletons developed what biologists call an arms race between predators and prey. As organisms were becoming increasingly complex. Better defence and attack mechanisms were needed to survive in the dangerous ecosystem, leading to the finetuning and development of new body shapes and organs, which in turn increase the complexity in a never-ending cycle.

Aliens on Earth

Among the creatures that existed in these times, we can find icons of palaeontology, monsters of the abyss and our very own ancestors. Trilobites, perhaps the quintessential prehistorical genus of animals, started to creep the seabed during the Cambrian. They would end up having a long run, as they continuously appear on the fossil record until the devastating Permian extinction c. 252 million years ago.

There were also creatures born out of nightmares including Anomalocaris, the big beast of the Cambrian oceans. This many- times-removed cousin of modern insects and crustaceans resembled a lobster with a disk-shaped mouth and two fang-like protuberances sticking from its face. While by modern standards it would have been fairly tame, at 1 meter long it probably was near the top of the trophic chain at the time. Other beasts roaming the seas include the weirdly shaped Opabinia, with its elephant like trunk and multiple eyes, and Odontogriphus, a flat bodied animal that counts octopuses and snails among its currently living relatives.

The Cambrian also saw the appearance of the earliest members of the phylum Chordata, distant grandparents of the human species. Pikaia did not resemble a modern vertebrate in its body shape, long and slim like a worm, but it possessed segmented muscle blocks, and most importantly, a notochord (a very primitive vertebral column). It vaguely looked like a lamprey, which is considered today to be the least developed vertebrate. Thus, the fossils of Burgess Shale have clues that help us elucidate our roots.

There another handful of periods of biological diversification after the Cambrian, but none produced the amount of phyla recorded then. Using a biblical metaphor, it was the fifth day of Creation, when the oceans became rich in the strangest creatures. The Cambrian explosion shows the powerful tool that is natural selection, a tool that allows life to overcome even the hardest ecological challenges, and that ended up producing us.

Juan Gorrochategui is studying for a BSc in Chemistry at Imperial College London

Banner Image: Tree of Life, Wikimedia Commons