[Polybius]

By biological definition, if two organisms can mate and produce fertile offspring, they belong to the same species.

[ValerieUSA]

produce fertile offspring

But could infertile offspring leave behind fossils just to confuse us???

[ValerieUSA]

After all... fossils would be their ONLY legacy of the infertile beings, while the others have US as their legacy.

[mamelukesabre]

By biological definition, if two organisms can mate and produce fertile offspring, they belong to the same species.

~~~~~~~`

There are instances where mules are able to procreate. It doesn't happen often, but it is possible.

[Right Wing Professor]

By biological definition, if two organisms can mate and produce fertile offspring, they belong to the same species.

Not really. For example blue and golden winged warblers are separate species, but in the zone where they overlap, they produce fertile F1 hybrid offspring. 'Species' is one of those categorizations that resists being defined too narrowly.

[Cleburne]

Not always. Some seperate species can interbreed and produce viable, fertile offspring, and hybrid species can arrise from crosses of two species. Then again, I wouldn't say the distinction between species, sub-species, and variety is always clear-cut- it's not. I suspect Neanderthals to perhaps merely be a subspecies or even just a variety.