Breeding with Neanderthals may have had a heavy price for early humans, according to a new study published recently in the journal GENETICS.

Harmful mutations present in the genome of Neanderthals made them up to 40% less fit reproductively than modern humans, according to the study. Although most of the effects have since been lost to time, these mutations likely passed to non-African humans when they interbred with Neanderthals. It is suggested that the mutations could still be affecting the fitness of some populations today.

The study was led by Kelley Harris of Stanford University, along with her colleague Rasmus Nielse, from the University of California Berkley and Copenhagen University.