In the geologic timescale, the "Cambrian period" refers to the lowermost densely fossil-bearing rocks. These rocks record the sudden appearance of creatures, with representatives of almost every living phylum found fully formed and with no signs of evolutionary transition, which is an enigma for evolution.
Yet Darwin admitted that the fossil record below the Cambrian strata seemed to be bereft of such creatures. Instead “species belonging to several of the main divisions of the animal kingdom suddenly appear in the lowest known fossiliferous rocks”—without any evidence of prior ancestral forms.
Darwin frankly acknowledged that this lack of ancestral forms was “a valid argument” against his theory. But he hoped that time—and more research—would provide the evidence that was lacking. Some 150 years later, the documentary Darwin’s Dilemma probes how Darwin’s dilemma has been aggravated—not resolved—by the last century of fossil discoveries, starting with the strange and wonderful creatures uncovered a century ago in the Burgess shale in British Columbia, Canada.
A new study led by University of Adelaide researchers has estimated, for the first time, the rates of evolution during the "Cambrian explosion" when most modern animal groups appeared between 540 and 520 million years ago, resolving "Darwin's dilemma" --the sudden appearance of modern animal groups in the fossil record during the early Cambrian period. In this study the scientists estimated that rates of both morphological and genetic evolution during the Cambrian explosion were five times faster than today - quite rapid, but perfectly consistent with Darwin's theory of evolution.
"It was during this Cambrian period that many of the most familiar traits associated with this group of animals evolved, like a hard exoskeleton, jointed legs, and compound (multi-faceted) eyes that are shared by all arthropods. We even find the first appearance in the fossil record of the antenna that insects, millipedes and lobsters all have, and the earliest biting jaws." says co-author Dr Greg Edgecombe of the Natural History Museum.
"The abrupt appearance of dozens of animal groups during this time is arguably the most important evolutionary event after the origin of life," says lead author Associate Professor Michael Lee of the University of Adelaide's School of Earth and Environmental Sciences and the South Australian Museum.
"These seemingly impossibly fast rates of evolution implied by this Cambrian explosion have long been exploited by opponents of evolution. Darwin himself famously considered that this was at odds with the normal evolutionary processes.
"However, because of the notorious imperfection of the ancient fossil record, no-one has been able to accurately measure rates of evolution during this critical interval, often called evolution's Big Bang.
The team, including researchers from the Natural History Museum in London, quantified the anatomical and genetic differences between living animals, and established a timeframe over which those differences accumulated with the help of the fossil record and intricate mathematical models. Their modelling showed that moderately accelerated evolution was sufficient to explain the seemingly sudden appearance of many groups of advanced animals in the fossil record during the Cambrian explosion.
The research focused on arthropods (insects, crustaceans, arachnids and their relatives), which are the most diverse animal group in both the Cambrian period and present day.
The Daily Galaxy via University of Adelaide