Scientists have discovered more than 700 fossils in southwestern China's Yunnan province that reveal complex animal life evolved millions of years earlier than previously thought, according to research published in the journal Science.

The fossils, dating to 539 million years ago during the late Ediacaran period, show evidence of three-dimensional animals that could move vertically through water and actively feed. These characteristics were previously believed to have first appeared at least 4 million years later during the Cambrian period, known for its explosion of complex animal life.

The discovery was made near a UNESCO World Heritage site in an exposed roadside area where different geological layers allow researchers to examine various time periods. The fossils include both remnants of earlier life forms that eventually disappeared and early examples of organisms that would evolve into modern animals.

Particularly significant among the findings are animals with bilateral symmetry - bodies that are essentially the same on left and right sides, along with distinct head and rear ends. Nearly all animal life on Earth today shares these basic body plan features. Previous research had found fossil tracks suggesting such animals existed, but the actual creatures had not been discovered until now.

The discovery helps resolve a long-standing debate in paleontology between genetic analysis suggesting earlier evolution of complex traits and fossil evidence that appeared to support later development. Genetic studies had indicated that humans and starfish shared a common ancestor during the Ediacaran period, but fossil evidence was lacking.

The research team believes this rapid diversification of animal life was made possible by rising oxygen levels in Earth's atmosphere and key genetic developments that had accumulated over time. Life on Earth began approximately 3 billion years ago, but complex animals did not develop until 2.4 billion years later, after which they rapidly multiplied and diversified.