Canadian Derek York at the University of Toronto devised a way to use lasers to melt the rocks. As York explains, a given sample of volcanic rock crystals will have contaminants - crystals of a much younger or older age that look exactly the same as the majority of the crystals. The contaminants skew the average age of the sample. By melting the crystals individually, scientists can screen out any contamination that might throw off the age. Walter and his team are still in the early stages of dating the stone tools found in Eritrea, and have made an estimate of their age based on the animal fossils they found - what are called index fossils.
Pigs evolve quickly, geologically speaking.
If a species of pig is identified and dated at one site, and then that same species is found at another, undated site, then the age of the new site can be estimated by correlation. Pig tooth found at the Dandero site. Image courtesy Bob Walter. The next thing to do is find enough volcanic rocks mixed into the fossil-bearing sedimentary rocks to come up with exact dates. For more pictures of the find, check out the slideshow .