Scientists have uncovered the earliest evidence of a complex brain in a 520 million-year-old fossil arthropod.
The well-preserved three-inch long creature belongs to the large family that includes spiders, insects and crustaceans.
Its discovery shows that anatomically complex brains evolved earlier than was previously thought.
The fossil, belonging to the species Fuxianhuia protensa, has a segmented flexible body and a large head.
Scientists found it embedded in mudstones in Yunnan Province, China, deposited during the "Cambrian explosion".
This was a period when life on Earth suddenly "exploded" into a myriad different forms.
Professor Nicholas Strausfeld, from the University of Arizona, US, who studied the fossil, said: "No-one expected such an advanced brain would have evolved so early in the history of multicellular animals."
He told how he spent five hours scrutinising the head and brain of Fuxianhuia under a microscope. His analysis revealed a number of "neuropils" - specialised regions of the arthropod brain with particular functions. Fuxianhuia had three optic neuropils on each side of its brain linked to its bulging eyes.
The brain looked surprisingly like that of a present-day insect. "In principle, Fuxianhuia's is a very modern brain in an ancient animal," said the professor.
The findings, reported in the journal Nature, support the idea that once basic brain design had evolved, it changed little over hundreds of millions of years.
"It's remarkable how constant the ground pattern of the nervous system has remained for probably more than 550 million years," said Prof Strausfeld. "The basic organisation of the computational circuitry that deals, say, with smelling, appears to be the same as the one that deals with vision, or mechanical sensation."
Fuxianhuia was a predator with eyes on the ends of stalks which could be rotated in different directions.
An X-ray technique was used to examine how details of the creature's brain and optic tracts were preserved.
Dr Greg Edgecombe, from the Natural History Museum in London, who also took part in the research, said: "While we've known about Fuxuianhuia for a while, the structure of its brain has never really been sorted out.
"We've found that it had three sections to its brain that connect to its head segments, so we can trace tracts from the brain that continue into the eyes and the antennae. These nerves are what we'd expect to see in living arthropods today, but this specimen is the first and earliest fossil example with this degree of preservation."
Dr Xiaoya Ma, also from the Natural History Museum, said: "We were surprised to find what we did because we would usually expect to see a primitively simple brain in these animals. But Fuxianhuia has a degree of complexity we would normally associate with living insects, and crustaceans such as lobsters and crabs, meaning that the complex nervous system already existed over half a billion years ago."