Life became more diverse and abundant in the seas during the Cambrian time period, which started about 545 million years ago. Fossils in Pre-Cambrian rocks are of simple life forms such as bacteria, with more complex soft-bodied creatures appearing towards the beginning of the Cambrian. Cambrian rocks show large numbers of many different types animals, many with hard shells.
This important shift is often described as an "explosion", but new evidence suggests that it may have been a more gradual change. Part of the difficulty is that the fossil record is not complete - some life forms were more likely to be fossilised than others, and the conditions that allow fossilisation to occur were also not constant.
Image: Trilobite fossils. Trilobites appeared on Earth during the Cambrian period. (credit: Sinclair Stammers/SPL)
Could the rise of creatures with eyes have caused an evolutionary arms race?
Professor Brian Cox visits the Burgess Shale, a very important fossil field in Yoho National Park in the Canadian Rocky Mountains. The fossils record the explosion of life that started about 540 million years ago during the Cambrian period. He explains a speculative theory that the evolution of creatures with eyes, such as trilobites, triggered the evolution of more complex forms of life.
The Cambrian explosion, or less commonly Cambrian radiation, was the relatively short evolutionary event, beginning around 542 million years ago in the Cambrian Period, during which most major animal phyla appeared, as indicated by the fossil record. Lasting for about the next 20–25 million years, it resulted in the divergence of most modern metazoan phyla. Additionally, the event was accompanied by major diversification of other organisms.[note 1] Prior to the Cambrian explosion,[note 2] most organisms were simple, composed of individual cells occasionally organized into colonies. Over the following 70 to 80 million years, the rate of diversification accelerated by an order of magnitude[note 3] and the diversity of life began to resemble that of today. Many of the present phyla appeared during this period, with the exception of Bryozoa, which made its earliest known appearance in the Lower Ordovician.
The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the “Primordial Strata” was noted as early as the 1840s, and in 1859 Charles Darwin discussed it as one of the main objections that could be made against the theory of evolution by natural selection. The long-running puzzlement about the appearance of the Cambrian fauna, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid change; and what it would imply about the origin of animal life. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures remaining in Cambrian rocks.
Phylogenetic analysis has been used to support the view that during the Cambrian radiation, metazoa evolved monophyletically from a single common ancestor: flagellated colonial protists similar to modern choanoflagellates.