WASHINGTON—New fossil evidence of the earliest complete skeleton of an ancient primate suggests it was a hyperactive, wide-eyed creature so small you could hold a couple of them in your hand — if only they would stay still long enough.

The 55-million-year-old fossil dug up in central China is one of our first primate relatives and it gives scientists a better understanding of the complex evolution that eventually led to us. This tiny monkey-like creature weighed an ounce or less and wasn’t a direct ancestor. Because it’s so far back on the family tree it offers the best clues yet of what our earliest direct relatives would have been like at that time, according to a study published Wednesday in the journal Nature.

“It’s a close cousin in fact,” said study author Christopher Beard, curator at the Carnegie Museum of Natural History in Pittsburgh. He said it is “the closest thing we have to an ancestor of humans” so long ago.

Primate is the order of life that includes humans along with apes, monkeys, and lemurs. Humans and other primates are set apart from other mammals because of our grasping five fingers and toes, nails, and forward-facing eyes. And this new species called Archicebus achilles fits right in, Beard said.

Among primates there are three suborders: anthropoids, which include apes, monkeys and us; and two other suborders that include lemurs and the lesser known tarsiers. This new species is in the same grouping as tarsiers, but close to the offshoot branch in the family tree where humans come from. The fossil includes anthropoid-like features.

“It’s a cute little thing; it’s ridiculously little,” Beard said. “That’s one of the more important scientific aspects of the whole story.”

With a trunk only 2.8 inches long, the furry creature was about as small as you can get and still be a mammal, Beard said. Just like elephants and horses, the farther back in time you get for some of today’s bigger mammals, the smaller they get, Beard said.

Because it was so small and warm-blooded, it had to eat bugs and move constantly to keep from losing internal heat, Beard said.

That means, Beard said, our earliest primate relatives were “very frenetic creatures, anxious, highly caffeinated animals running around looking for their next meal.” They lived in a tree-lined area near a Chinese lake, swinging around trees in a hotter climate, Beard said.

Outside experts praised the study as significant, confirming what some thought about our primate ancestors. Rick Potts, director of the human origins program at the Smithsonian Institution, said this fossil’s mix of different features illustrate the fascinating and crucial changes that occur around major evolutionary branch points in our family tree.

The study also bolstered another theory that early primates first developed in Asia, even though humans evolved nearly 50 million years later in Africa, Beard said.