Cro-Magnon are modern humans that appeared during the Pleistocene.
In the Series 3 Finale Mammoth's Undertaking Journey Allen and other tries to protect a herd of Woolly Mammoths from these hunters. Cro-Magnons also happen to be Allen's ancestors, and he invited some into the park to be zookeepers. They needed some help, but they became very good at it, and became friends with Allen.
Cro-Magnons are one of the last humans to brought to the park the other being Homo Neanderthalensis.