No one knows how her body found its way into the stream or how long her distraught parents may have searched the shallows for the missing 3-year-old.

The child’s fossilized skeleton — a tiny skull, a jaw with baby teeth intact, a clutch of finger bones, the curled commas of ribs — are remains of a domestic calamity 3.3 million years ago when the human family was in its infancy, so long ago that the river in which she may have drowned has itself turned to stone.

Discovered in Ethiopia, her skeleton is the most ancient complete set of infant remains on record, at least 3 million years older than any other comparable fossil of childhood, scientists announced yesterday in the journal Nature.

The tiny female was the child of an ancestral pre-human species called Australopithecus afarensis, the same species as the iconic fossil specimen Lucy. Their species thrived in East Africa between 3 million and 4 million years ago. Modern humankind, by comparison, arose just 200,000 years ago. The child’s bones are yielding rare insights into the origins of upright walking, brain development, the beginnings of speech, and the unique pace of childhood development that sets humankind apart from all other primates, the researchers said.

With the shoulders of a young gorilla and legs jointed more like a human girl, her bones merge the anatomy of humanity’s most ancient ancestors with more contemporary human characteristics, several experts said. She may have deftly swung from branches but also easily walked erect, the fossils suggest.