Animal interaction behind ‘Cambrian Explosion’?

5 min read

‘Missing’ ancestors of today’s animals may not be missing after all

An event as simple as the world’s first bite may have sparked an ancient “explosion” of life 500 million years ago that led to the rise of the broad groups of animals that are still alive today.

The cause of what is known as the “Cambrian Explosion” — which occurred during the Cambrian Period 542 million to 490 million years ago — has puzzled scientists for years. Theories about the event’s cause include an increase in the amount of atmospheric oxygen, a recovery from a global glaciation, and key genetic changes in precursor animals that allowed the development of bilateral symmetry, hard shells and bones, and rapid locomotion.

Harvard Professor of Biology and of Geology Charles Marshall presented his alternate theory Tuesday (April 29), suggesting that it was an increase in interactions between species, such as predation, that drove an escalating evolutionary process that led to the development of teeth and claws and the wide variety of characteristics that we see among Earth’s animals today.

The Cambrian Explosion was unique, Marshall said, because, though there have been mass extinctions — such as that of the dinosaurs — and recoveries since, there has never been another event as sweeping as that which occurred in the Cambrian seas 500 million years ago. It was during that time when all the modern phyla of animals first arose. Phyla are major classifications of life that include broad groups of creatures. The phylum Chordata, for example, includes all vertebrates, such as mammals, reptiles, amphibians, and birds.

Marshall, who spoke in the Geological Lecture Hall Tuesday evening as part of the Harvard Museum of Natural History’s annual lecture series, started off his talk by disputing whether the event commonly termed an “explosion” was rapid enough to earn that moniker. The rise of modern animal groups happened over millions of years. Only looking back over 500 million years of history could the expansion of life that occurred be termed an “explosion,” Marshall said.

Prior to the Cambrian Explosion, life on land was unknown and life in the sea was made up of soft-bodied multicellular creatures that strained food from the seawater around them or fed on mats of bacteria on the ocean floor. Though these animals virtually disappeared at the beginning of the Cambrian, the ancestors of the new modern phyla that replaced them haven’t been found.

Marshall reviewed other theories explaining the explosion and said that though each of them has merit — an increase in atmospheric oxygen, for example, would be needed to fuel more active lifestyles — each also falls short in some way.

As Marshall pondered alternatives, he began to think that it was possible that the creatures in the pre-Cambrian seas during the Ediacaran Period didn’t entirely disappear. Though they were very different from what followed, they may have been genetically complex enough to hold the genetic seeds of the explosion.

Marshall cited recent findings from genetic studies that indicate even creatures as diverse as flies and fish share many of the same genes. They differ, he said, more in how the genes are used — whether they’re switched on or off — than in the genes’ presence or absence.

“It’s not new genes that create new morphological innovation, but rather the way they’re wired together,” Marshall said. “[Different-looking creatures] are not apples and oranges.”

If the precursors to the creatures that arose during the Cambrian Period were swimming in the Ediacaran seas, something had to spark the dramatic change.

Marshall said that computer modeling of the forms that plants would take under different environmental conditions provided a clue. The models showed that widely divergent plants can result from a simple ancestor whose descendants are subjected to different environmental conditions. The model started with a simple primitive plant form and applied six basic genetic rules. It then added four selective pressures to drive evolutionary change — reproductive success, mechanical stability, light interception, and minimized surface area. The model produced 20 widely different body types. When researchers checked the fossil record, they found all types represented.

Applying that lesson to animals, Marshall began to search for an environmental force that might have driven such dramatic change in the fleshy animals that populated the oceans before the Cambrian began. Marshall realized that those creatures had no organs of interaction — no eyes, no antennae, no jaws or claws — and began to think that the new force on the scene was the ability of animals to interact with each other.

“Ediacarans were not interacting with each other as animals do today,” Marshall said. “I think what drove the Cambrian Explosion was ecological interactions.”

The other factors that have been cited as playing a role in the Cambrian Explosion very well may have had a hand, Marshall said, but they made the conditions ripe for the change driven by interactions among animals. Just what the trigger was that sparked those changing interactions, Marshall didn’t know, but, in a world populated by what he described as fleshy “beefsteaks” lying on the ocean floor, it may have been something as simple as the evolution of jaws with toothlike projections that allowed the world’s first painful bite.

“I believe … the explosion was driven by the onset of adult-adult interactions,” Marshall said. “Maybe the evolution of jaws or a large enough gut, or the evolution of something like chitin so they could bite rather than just giving a nasty suck.”