For more than a century, scientists have searched for fossils that offer new clues as to when and how our earliest ancestors split off from their invertebrate brethren and began developing bones. But finding intact early vertebrates, much less well-preserved invertebrates, is exceedingly difficult -- their soft bodies easily malformed by the pressures shifting rocks and weather.

But there is hope, thanks to a dig conducted by paleontologists in 2012. The treasure trove of fossils discovered in southwestern Canada is finally beginning to offer new insight into that special moment in biologic time -- when vertebrates first emerged.

The most significant species found in the collection of fossils -- which are more than 500 million years old -- is a two-inch fish-like vertebrate known as Metaspriggina.

The species roamed the primordial seas between 543 million to 493 million years ago, a period known as the Cambrian Explosion, an evolutionary "big bang," when almost all evidence of complex life begins to appear.

The Metaspriggina fossils, along with a few thousand more representing 54 other species, were unearthed by Jean-Bernard Caron, a paleontologist at the Royal Ontario Museum in Toronto. Metaspriggina fossils had been discovered before, but never in such abundance and never as well preserved as these.

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. This early vertebrate was something of a mystery for years, known only from a pair of ambiguous fossils.

Two years after Caron's discovery, detailed analysis of the Metaspriggina fossils puts the tiny fish at or near the top of the vertebrate family tree -- meaning all fish, mammals, and birds evolved from it.

"It's clearly a benchmark early vertebrate, which we haven't had before," Thurston Lacalli -- a researcher at the University of Victoria in British Columbia, who was not involved in the study -- told the New York Times.

One of the keys to placing the Metaspriggina at the top of the tree is its gills, which scientists say were primed and ready to evolve into jaws.

Even before Metaspriggina was discovered, scientists had predicted this is what the last jawless vertebrates looked like, just before they split off and developed chompers.

"For the first time, we are able to say this is really close to this hypothetical ancestor that was drawn based on a study of modern organisms in the 19th century," said Caron.

The new study, detailing Metaspriggina, was published this week in the journal Nature.