The molecular clock typically assumes steady genetic change, but molecular dates often predate the earliest fossils by millions of years. Graham Budd and Richard Mann propose the Covariant Evolutionary Tempo model, in which evolutionary rates accelerate during explosive early radiations of large clades, producing apparent time gaps. If true, these bursts could account for a roughly 30-million-year mismatch between molecular estimates and fossils such as Treptichnus, though the idea still needs empirical testing.
New Model Says Evolution's Molecular Clock May Speed Up During Bursts — Could That Close a 30‑Million‑Year Gap?
We May Have the Evolutionary Timeline All WrongDonald Iain Smith - Getty Images
The molecular clock — the idea that genetic changes accumulate at a roughly steady rate — underlies many estimates of when major evolutionary events occurred. But a persistent mismatch between molecular dates and the fossil record has prompted new thinking about whether that clock always ticks at a constant rate.
Using molecular-clock methods, researchers infer that complex animals may have arisen roughly 30 million years before the earliest commonly cited animal fossils, such as worm-like trace fossils attributed to the genus Treptichnus (dated near 538 million years ago). Some of that discrepancy could reflect incomplete fossil preservation or tiny early animals that left few traces, but other scientists now question the uniform-rate assumption itself.
In a paper in Systematic Biology, palaeontologist Graham Budd (Uppsala University) and mathematical ecologist Richard Mann (University of Leeds) propose the Covariant Evolutionary Tempo model. Rather than assuming a single, steady substitution rate across time and lineages, their model allows evolutionary rates to covary with diversification: major clades can experience rapid early radiations accompanied by elevated rates of molecular evolution.
“This model predicts that diversity is dominated by a small number of extremely large clades at any historical epoch; that these large clades are expected to be characterized by explosive early radiations accompanied by elevated rates of molecular evolution; and that extant organisms are likely to have evolved from species with unusually fast evolutionary rates,” the authors write.
Explained in plain terms (via an interpretation by evolutionary biologist Max Telford of University College London), the idea is that when a successful, broad group of organisms first diversifies, the rate of genetic change can accelerate. For a simple analogy: if a constant-clock interpretation would assign one genetic substitution per million years between two species, a period of accelerated change could produce several substitutions in a much shorter interval, making molecular methods infer an older split than actually occurred.
Applied to the early history of animals, these bursts of fast molecular evolution during the initial radiation of large clades could make molecular estimates appear older and thereby reduce the apparent 30-million-year gap between genetic inferences and the earliest fossils.
Crucially, Budd and Mann — and commentators such as Telford — emphasize that the Covariant Evolutionary Tempo is a hypothesis that requires empirical testing. If supported, it would reshape how scientists calibrate molecular clocks and interpret deep-time divergences, with implications for reconciling genetic and paleontological timelines.
Bottom line: Allowing for variable rates of molecular evolution during explosive early radiations offers a plausible and testable mechanism to narrow discrepancies between molecular dates and the fossil record, but further data and analysis are needed to confirm the idea.
Help us improve.
