Why are so many of us scared of spiders? Are we born with arachnophobia, or do we develop it during our childhood as we observe how adults and other children around us react? Is it a nature or nurture phenomenon? Experts appear to disagree.

Two American researchers believe that our fear of spiders is now part of our DNA as a product of human evolution. Since our ancient ancestors started walking on two legs in Africa about two million years ago, we gradually developed the survival instinct to avoid spiders--a trait that became ingrained in our genetic makeup.

They say that in prehistoric times there were many more poisonous spiders in Africa compared to today.

Prof. New and Prof. Tamsin German, who works at the University of California at Santa Barbara, recently carried out a study to determine how good modern humans are at identifying spiders when presented with several different images of nasty things at the same time.

Their findings have been published in the academic journal Evolution & Human Behavior.