Cambrian explosion

The Cambrian explosion refers to the relatively sudden emergence of many forms of life on Earth about 543 million years ago. The fossil record shows that within the span of twenty to thirty million years—a short period in evolutionary terms—millions of species evolved in the planet’s oceans. The distant ancestors of many animal species had their origins during this period. The cause of this event has remained a mystery to paleontologists, although there have been several possible theories. These have included the thawing of a frozen Earth, an increase in oxygen levels in the oceans, and the evolution of improved senses.

Background

The earliest life-forms on Earth were single-celled bacteria that scientists believe first appeared about 3.8 billion years ago. These bacteria slowly evolved during the next few billion years, acquiring the ability to photosynthesize energy from sunlight and developing into more complex multicellular organisms. About 700 million years ago, these organisms evolved into the first animals—tiny sponges that would eventually lead to the development of worms, anemones, and jellyfish.

By about 543 million years ago, life on Earth was confined to the oceans, a low-oxygen environment where simple, soft-bodied creatures fed upon a thick layer of microbial slime on the sea floor. Then, over the course of the next twenty or thirty million years, the Earth’s oceans seemingly erupted in biological diversity. In an evolutionary blink of an eye, the seas were filled with millions of complex multicell organisms. Many of these species featured advanced biological developments that eventually led to the evolution of almost all modern-day animal species. Scientists refer to this event as the Cambrian explosion, named after the Cambrian period.

Among the notable evolutionary leaps during the Cambrian explosion were the brachiopods, hard-bodied creatures with protective shells that resemble clams; chordates, animals that developed a dorsal nerve cord; and the arthropods, animals with legs and compound eyes that were the ancestors of modern spiders, insects, and crustaceans. Another key biological advancement was the evolution of mobile predators such as the Anomalocaris, a three-foot-long hunter that consumed its prey with its tooth-filled jaws.

Some species proliferated in the Cambrian seas and evolved into the first vertebrates—animals with backbones—and the trilobite, an insect-like arthropod with a complete exoskeleton covering its body. Other species developed biological traits that did not survive the evolutionary process and died out after a few million years. One of these was the Opabinia, a five-eyed arthropod with an elephant-like “trunk” extending from its head. The Cambrian period ended about 490 million years ago with a mass-extinction event that wiped out about 40 percent of all life on Earth. The exact cause of the extinction is unknown, although scientists theorize that the ocean’s oxygen levels may have been depleted or the planet may have entered a period of severe global cooling.

Overview

Paleontologists have had several theories as to what caused the Cambrian explosion. One of the most common factors seems to be that an increase in the oxygen levels of the Earth’s oceans triggered the sudden burst of biodiversity.

The Precambrian seas had very little oxygen. The creatures that thrived in this environment tended to be simple organisms that relied on carbon dioxide, sulfur, or iron molecules to break down food. Larger, more mobile animals need oxygen to generate sufficient energy to power their bodies. Evidence suggests that a gradual and incremental increase in the oxygen levels of the Earth’s oceans began about 800 million years ago and approached modern levels by the start of the Cambrian period. This increase may have provided an evolutionary boost that led to the development of more complex species.

One theory on the source of this oxygen is that the planet was covered with a thick layer of ice beginning about 750 million years ago. This period, referred to as Snowball Earth, ended about 635 million years ago, after which the melting ice released large amounts of oxygen into the atmosphere and oceans. Running water from the thawing surface may have also washed minerals such as calcium into the seas, possibly spurring the development of hard shells. Some scientists have disputed this idea, however, claiming the timeframe of the melting occurred too early to lead to the Cambrian explosion. While some have argued the rise in oxygen levels was too small and incremental to spark such a rapid evolutionary event, still others have contended that less oxygen may have been needed than traditionally believed necessary.

Another school of thought is that evolutionary biology fueled the process, providing a genetic blueprint for more efficient life-forms. Cambrian animals began exhibiting bilateral symmetry, a biological trait in which the body is divided into two identical halves. This shape tends to lead to the development of a front and a back, with sensory organs such as the mouth and eyes forming at the front—a body type that is also conducive to the formation of a brain. Creatures with this characteristic were able to move through the water faster and consume food more easily, thereby increasing their chances of survival. When some of these species evolved into predators, it would have spurred their prey into evolving defensive mechanisms, touching off a biological revolution.

There are some scientists who have even questioned whether the Cambrian explosion was as dramatic as the fossil record seems to indicate. They have suggested that many evolutionary milestones had been reached millions of years earlier and were well underway at the beginning of the Cambrian period. The reason it appears that a biological leap occurred at that boundary was that prior to that, life-forms did not possess hard-shelled bodies. The soft tissues of earlier species would not have been preserved as fossils; therefore, when they died, they would have left little trace. There has been some indication that more complex life-forms existed as early as 575 to 630 million years ago, but that they mysteriously became extinct. This extinction may have also removed an evolutionary roadblock for the species of the Cambrian period, leading to their proliferation.

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