The earliest known fossilized remnants of sophisticated organisms make a sudden, almost inexplicable appearance in geological strata dating back 538 million years.
Among the very first of these are rudimentary fossilized impressions, identified as Treptichnus, which were produced by a worm-like entity possessing distinct anterior and posterior regions. A multitude of other fauna subsequently emerge with remarkable speed, representing the progenitors of contemporary animal lineages: ancient arthropods akin to crustaceans, shelled gastropods, and the ancestral forms of echinoderms like starfish and sea urchins.
The swift proliferation of such disparate animal forms, particularly their conspicuous absence in strata of slightly lesser age, posed a significant challenge to Charles Darwin’s hypothesis of gradual evolutionary progression, and it has continued to perplex researchers for generations.
However, a recent scholarly publication may offer a compelling resolution.
In his seminal 1859 work, On the Origin of Species, Darwin mused: “If my theory be true … during these vast … periods of time, the world swarmed with living creatures. To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer.”
Currently, scientific consensus is divided regarding the precise timing of these ancient animal evolutions. The crux of the issue lies with a scientific methodology developed in the late 20th century, known as the molecular clock.
As elucidated in my publication, the Tree of Life, the molecular clock operates on the premise that genetic mutations accumulate at a consistent rate, analogous to the rhythmic ticking of a pendulum clock.
Assuming this principle holds, quantifying the genetic dissimilarities between any two species enables us to infer their degree of relatedness and, consequently, the age of their common ancestor.
For instance, the divergence of human and chimpanzee lineages is estimated to have occurred approximately six million years ago. Should a particular chimpanzee gene exhibit six genetic variations compared to its human counterpart, and assuming a uniform rate of genetic change, this would suggest that each genetic difference corresponds to one million years of evolutionary separation.
The molecular clock methodology is theoretically capable of dating evolutionary events across the entire spectrum of life.
Initial applications of molecular clocks by zoologists yielded the
extraordinary inference that the common ancestor of all complex animal life existed as far back as 1.2 billion years ago. More refined analyses have since provided more plausible estimates for the age of this ancestral animal, placing it around 570 million years ago.
However, this figure remains approximately 30 million years anterior to the earliest fossil evidence.
This aforementioned 30-million-year temporal discrepancy is, in fact, somewhat advantageous for Darwin’s framework. It suggests ample time for the ancestor of complex organisms to evolve and subsequently diversify, giving rise to new species that natural selection could gradually mold into forms as distinct as fish, arthropods, mollusks, and echinoderms.
The conundrum presented by this ancient estimated origin is the implication that a diverse array of prehistoric fauna must have inhabited these ancient oceans for 30 million years without leaving any discernible fossil traces. While researchers anticipate gaps in the fossil record, this particular void would represent a substantial omission.
A prevalent hypothesis to account for the absence of these early fossils posits that for a protracted period of 30 million years, complex animals were diminutive and lacked rigid structures, rendering them exceptionally difficult to preserve as fossils. Subsequently, around 540 million years ago, according to this theory, these minute organisms began to increase in size, possibly driven by rising oxygen levels.
It is this inferred augmentation in physical scale that some scientists have advanced as an explanation for the abrupt emergence of complex fauna in the fossil record.
A recent research paper authored by paleontologist Graham Budd and mathematician Richard Mann proposes an alternative interpretation for the temporal disparity between the ancestral age inferred by molecular clock data and the more abrupt, later appearance of complex fossilized remains. Budd and Mann contend that the molecular clock’s tempo may not be as uniformly consistent as previously assumed.
The novel concept introduced is that the rate of evolutionary change accelerates significantly at the inception of major organismal lineages.
To revisit our earlier illustration, imagine that for a span of several million years, our hypothetical clock’s rate doubled, ticking twice per million years instead of once. An accelerated ticking rate would create the illusion of greater temporal passage, akin to fast-forwarding a video, thereby pushing the estimated origin of the animal ancestor further into the past.
A more rapid rate of genetic modification would also facilitate quicker anatomical diversification among these organisms. This resolves Darwin’s quandary by making it more plausible for the diverse branches of the animal kingdom to diverge. The initial animal ancestor could rapidly differentiate into distinct groups such as vertebrates, mollusks, arthropods, and echinoderms.
The overarching implication of this new hypothesis is to align the estimated age of the common animal ancestor more closely with the epoch of its immediate descendants’ appearance in the fossil record.
While the accelerated clock hypothesis requires empirical validation, it holds the potential to elucidate other discrepancies between molecular clock estimations and fossil evidence. It is conceivable, for example, that the earliest flowering plants existed for tens of millions of years prior to their eventual fossilization. Furthermore, it could contribute to resolving ongoing scientific debates concerning whether early primates, carnivores, and rodents coexisted with the final dinosaur species during the Late Cretaceous period.
Regarding the origins of animal life, at the very least, I am confident that Darwin would find this proposition agreeable.
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