The fossil record indicates a rapid emergence of complex animals approximately 538 million years ago. These ancient remains include simple marks attributed to worm-like organisms (Treptichnus), as well as ancestors of modern arthropods, molluscs, and echinoderms. The sudden appearance and diversification of these distinct animal groups, and their absence in older rock layers, presented a challenge to Charles Darwin's theory of gradual evolution.
The Molecular Clock and Evolutionary Timelines
One method used by scientists to estimate evolutionary divergence times is the molecular clock. This concept proposes that genetic changes accumulate at a relatively consistent rate over time. By comparing genetic differences between species, researchers aim to calculate the age of their last common ancestor.
Early applications of molecular clocks in zoology initially suggested that the common ancestor of all complex animals lived as far back as 1.2 billion years ago. Subsequent refinements to this methodology have yielded more recent estimates, placing the age of this animal ancestor at approximately 570 million years old.
The Discrepancy with the Fossil Record
This 570-million-year estimate creates a roughly 30-million-year gap between the predicted origin of complex animals and their first appearance in the fossil record (540 million years ago). This timeframe could theoretically accommodate a period of gradual evolution and diversification before fossilization occurred.
One prevailing explanation for this gap posits that early complex animals were small and soft-bodied, rendering them unlikely to fossilize. According to this theory, these organisms only began to increase in size and complexity around 540 million years ago, possibly due to rising oxygen levels, leading to their subsequent preservation in the fossil record.
A New Hypothesis on Molecular Clock Variability
Recent research by paleontologist Graham Budd and mathematician Richard Mann offers an alternative explanation. Their paper suggests that the molecular clock may not tick at a constant rate. They propose that evolutionary rates can accelerate during the initial emergence and diversification of a major group of organisms.
Under this hypothesis, a faster rate of genetic change during early evolutionary periods would make it appear as though more time had passed, effectively pushing back the estimated age of the animal ancestor when using a standard molecular clock. This accelerated evolution would also facilitate a more rapid diversification into distinct animal lineages, such as vertebrates, molluscs, arthropods, and echinoderms.
Implications for Evolutionary Science
This concept of a variable molecular clock could reconcile the age estimates from genetic data with the timing of fossil appearances, bringing the two lines of evidence into closer alignment. If substantiated, this idea could also help resolve other discrepancies observed between molecular clock predictions and the fossil record for various groups, including early flowering plants, primates, carnivores, and rodents.