In 2022, powerful X-rays turned a tiny “human ancestor” candidate into a spiny sea creature |
There was once an extraordinary little animal that fascinated the world of science when it was discovered as a possible origin point for the evolution of humans from rock formations in China. This tiny, spherical organism, which is called Saccorhytus coronarius and dates back to the early Cambrian, about 535 million years ago, is only one millimetre long, and at first glance, seemed to be an example of the origins of humans themselves.Peering beyond the compressed appearance of the fossil, researchers discovered anatomical features that contradicted earlier theories regarding the earlier anatomical interpretations. This finding suggested that the small marine creature should be excluded from being a direct ancestor to humans, leading to a broader reconsideration of how certain animal fossils are analysed. It highlighted the general difficulties of making sweeping claims about prehistoric creatures based on fossils that have undergone immense pressures over millions of years.Revealing the true anatomy beneath the surfaceThe initial excitement surrounding the specimen stemmed from the belief that it might represent a primitive member of the deuterostome group. This overarching evolutionary branch contains everything from starfish to humans. According to a Nature study under the title Saccorhytus is an early ecdysozoan and not the earliest deuterostome, earlier interpretations appear to have misread the superficial holes surrounding the creature’s massive mouth. What had previously been identified as primitive gill slits, a classic diagnostic feature of early vertebrate relatives, were interpreted by the new study as something entirely different.To resolve the controversy, an international team of researchers utilised a particle accelerator to generate high-intensity X-rays, capturing hundreds of highly detailed structural angles to reconstruct the organism in three dimensions. As the detailed analysis in Nature indicated, the digital reconstruction suggested that these supposed respiratory holes were likely the broken bases of sharp, protective spines that had snapped off during the fossilisation process. Without confirmed gill structures, the core argument for placing the animal on our own evolutionary path was significantly weakened, prompting scientists to look for an alternative classification.
This spiky marine creature is now believed to be an early ecdysozoan, highlighting the complexities of fossil interpretation and the scientific self-correction. Image Credit: Saccorhytus is an early ecdysozoan and not the earliest deuterostome Fig 4
Relocating a bizarre creature on the tree of lifeThe correction of the anatomical blueprint did not strip the microscopic organism of its historical significance. Instead, it helped clarify a long-standing mystery regarding the rapid diversification of marine life during the Cambrian explosion.By tracing the revised arrangement of the spines, researchers proposed realigning the creature with a different major animal group of ancient biodiversity. As documented in a study, Early evolution of the ecdysozoan body plan, Saccorhytus potentially belonged to the primitive lineage of ecdysozoans, a highly successful group of moulting animals that includes modern insects, spiders, and roundworms. Rather than being a distant grandparent to humanity, the spiny, sack-like animal appears to have been an early pioneer of an entirely separate biological dynasty.In fact, nowadays, the reassessment of Saccorhytus has been viewed as a notable case of scientific self-correction, illustrating how advancements allow humanity to get further insight into its distant geological past. Indeed, it shows us that theories about evolution benefit from being backed by physical evidence rather than assumptions. It suggests that our origins are rooted in the age when there were numerous experiments with different body plans, and that research often depends on the minutest details of those times.One might say that although popular science media tends to look for simple paths from primordial times to Homo sapiens, in reality, the fossil record often resembles an incredibly tangled forest, and even one small creature with its prominent mouth can present interpretive challenges for decades.
