Lahn offers an analogy: Medieval monks would copy manuscripts and each copy would inevitably contain errors — accidental mutations.

Years later, a ruler declares one of those copies the definitive manuscript, and a rush is on to make many copies of that version — so whatever changes from the original are in this presumed important copy become widely disseminated.

Scientists attempt to date genetic changes by tracing back to such spread, using a statistical model that assumes genes have a certain mutation rate over time.

how many years can carbon dating go back-41

That the defining feature of humans — our large brains — continued to evolve as recently as 5,800 years ago, and may be doing so today, promises to surprise the average person, if not biologists.

"We, including scientists, have considered ourselves as sort of the pinnacle of evolution," noted lead researcher Bruce Lahn, a University of Chicago geneticist whose studies appear in Friday's edition of the journal Science.

The key advantage is to require minute samples of fossil for the dating.

This technique was first implemented in France at the center of the low radioactivity of Gif-sur-Yvette in France with an instrument called Tandetron.

This assimilation stops upon the death of the organism, thus halting the absorption of any more carbon 14.

The atoms of carbon 14 then proceed to decay exponentially, with a half life of 5,700 years.Lahn and colleagues examined two genes, named microcephalin and ASPM, that are connected to brain size.If those genes don't work, babies are born with severely small brains, called microcephaly.The cosmic rays originating from the Sun collide with nuclei in the upper atmosphere and are capable of breaking off individual neutrons.These neutrons, once freed, can interact with atoms of nitrogen 14 in air, causing the expulsion of a proton and the formation of carbon 14.Using DNA samples from ethnically diverse populations, they identified a collection of variations in each gene that occurred with unusually high frequency.