This two-inch, 505-million-year-old creature belonged to the lineage that would later produce sharks, eels and other fish — along with birds, reptiles and mammals like us. This early vertebrate, known as Metaspriggina, was something of a mystery for years, known only from a pair of ambiguous fossils. But recently, scientists unearthed a trove of much more complete Metaspriggina fossils.

As they report today in the journal Nature, the new fossils offer a remarkably detailed understanding of the first vertebrates, helping scientists understand how major parts of our own anatomy — from eyes to jaws to our muscles — evolved.

“It’s clearly a benchmark early vertebrate, which we haven’t had before,” said Thurston Lacalli of the University of Victoria in British Columbia, who was not involved in the research.

Discovering the origins of vertebrates has occupied biologists for decades. A few living invertebrates, such as worm-like animals called lancelets, are closely related to vertebrates, but our ancestors split off from theirs more than 600 million years ago.