Darwin was once puzzled: why do animal fossils suddenly appear in great abundance in Cambrian strata, while their ancestors are almost nowhere to be found in older rocks? For over a century, this mystery has troubled paleontologists.

Now, an answer may be emerging. A research team led by Peiyun Cong from Yunnan University has discovered more than 700 exquisitely preserved soft-bodied animal fossils in eastern Yunnan, dating back approximately 554 to 539 million years. These fossils belong to the terminal Ediacaran Jiangchuan Biota. Unlike most Ediacaran fossil sites that preserve organisms as impressions on sandstone, these fossils are preserved as carbonaceous films, revealing fine internal anatomical structures such as the proboscis and digestive tract of worm‑like animals.

Fig. Artistic reconstruction of the Jiangchuan biota (drawn by Wang Xiaodong)

The team identified fossils from multiple animal groups, among which bilaterians — animals possessing a body plan with front‑back, left‑right, and dorsal‑ventral axes — dominate the assemblage. Bilaterians account for about 98% of all extant animal species, including the vast majority of animals, from insects to humans. Even more excitingly, the fossils include the oldest known deuterostomes, ancient relatives of chordates (the group to which humans belong), echinoderms, and hemichordates. This discovery demonstrates that a Cambrian‑like complex animal community had already emerged in marine ecosystems well before the Cambrian explosion.

These fossils fill a critical gap in the transition between the Ediacaran and Cambrian periods, providing the most substantial evidence to date for solving Darwin‘s dilemma. The findings were published on April 3, 2026, in the journal Science.

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Reference

Gaorong Li et al. , The dawn of the Phanerozoic: A transitional fauna from the late Ediacaran of Southwest China. Science392, 63-68 (2026). DOI:10.1126/science.adu2291