That is a necessary but not a sufficient condition, as the type of rock involved, called carbonaceous chondrite, is found in other asteroids, too.

ECONOMIST: Asteroid impacts

"If more data confirm Eros is primordial, Eros will be a link between the chondrite meteorites found on Earth and the history of the solar system's formation, " McCoy said.

CNN: Asteroid Eros resembles 'building blocks' of Earth

If the arrival was made of carbonaceous chondrite, it would have weighed 25 times as much as all the other carbonaceous chondrite material that had fallen over the previous half million years.

ECONOMIST: Paleoforensic science

Assuming that the figure was similar during the Cretaceous, it would be a long shot that the first fossil meteorite to turn up from 65m years ago was made of such a rock unless a huge extra supply of carbonaceous chondrite had suddenly arrived on the planet.

ECONOMIST: Paleoforensic science