In a fascinating twist in the understanding of animal evolution, newly discovered fossils indicate that the invertebrate ancestors of humans emerged much earlier than scientists previously believed. These findings reveal essential characteristics that link modern animals, including humans, to simple, worm-like organisms, reshaping the scientific understanding of early animal evolution and the timeline of life on Earth.
Animal life, incredibly diverse and complex, has colonized almost every environment on Earth, from the hostile hydrothermal vents deep in the sea to the skies crossing continents. However, the planet was not always teeming with complex animal life. For the first 3.7 billion years after its origin, life was small, simple, and largely confined to the oceans, in a world dominated by microbes and marked by significant climatic fluctuations. However, all this seems to have changed about 538 million years ago during the Cambrian Period, when the Cambrian Explosion occurred.
During this explosion, animals easily recognizable as living groups today appeared in the fossil record, from echinoderms and arthropods to various types of worms. This seemingly abrupt appearance of animals in a geological blink of an eye has intrigued scientists since Charles Darwin. Many of these new beings belonged to a group called bilateria, characterized by their symmetrical left and right sides, and which now includes all animals with brains and complex musculature.
A recent study, published in the journal Science, helps resolve the long-standing question of whether this impressive diversification occurred all at once during the Cambrian Explosion or if the ancestors of Cambrian and modern animal groups can be traced further back in time. According to recent revelations, the Ediacaran period, which preceded the Cambrian, was much more enigmatic, with organisms that defy traditional classifications.
These Ediacaran organisms, with strange bodies often resembling shapeless sacs or padded cushions, have no obvious counterparts among living species. However, recent evidence about their reproductive strategy and development suggests they were indeed animals, though very simple and without direct living descendants. It was only towards the end of the Ediacaran period that the fossil record began to suggest the presence of more complex and recognizable animals.
In a discovery that sheds light on this nebulous gap, Gaorong Li, then a Ph.D. student at the Key Laboratory of Palaeobiology in Yunnan, found a bizarre worm that lived anchored to the seafloor. Dubbed the bugle worm, this strange being could invert its proboscis to collect food, revealing itself as a complex animal, yet unlike any known today.
As more rocks were examined, it became evident that more animals were hidden in the Jiangchuan biota. In 2024, a team from the University of Oxford joined the exploration, discovering a new fossil community that included characteristic organisms from both the Ediacaran and Cambrian periods. Surprisingly, fossils previously known only from the time of the Cambrian Explosion were found, such as a primitive animal resembling Mackenzia and swimming predators called ctenophores.
The most impressive find was the earliest evidence of the group to which humans belong: the deuterostomes. Some specimens, with stems and tentacles, resemble known Cambrian fossils called cambroernids, showing that human evolutionary history has roots in the Ediacaran period. The discovery of complex animals in the Jingchuan biota suggests that several animal groups shared the world with the Ediacarans for millions of years, indicating that complex animal life has an older heritage than the Cambrian Explosion.
Original published at O Cafezinho.