An exciting discovery was made just this past week in the well-known Cerutti Mastadon site near San Diego, CA. Near to one of the mastodon skeletons was found also large stones that showed tell-tale signs to use as tools by early hominid visitors.
What is interesting is not that people lived near San Diego before we called it by that name, but rather that hominids arrived there a full 115,000 years earlier than archaeologists had ever expected. Meanwhile the mastadons, a slightly smaller version of the mammoth, were commonly found in the Americas 130,000 years ago. A natural question to ask is how do we know that this particular mastodon site really is that old?
Archaeologists determine ages by measuring the radioactive decay of certain elements like carbon or in this case uranium. Okay, fine, so where does the astronomy part fit in? Well in addition to the sunlight we appreciate so much in springtime, the sun also makes cosmic rays.
Although they sound rather exotic, cosmic rays are mostly just fast-moving protons. These protons strike the upper atmosphere of the Earth and in doing so turn some of elements radioactive. Later, these atoms that fall down to Earth and get incorporated into plants and animal matter. At the end of the day, we sit down to dinner and consume said food which by this time is a healthy mix of ordinary and radioactive materials.
When we humans die, or any animal dies, then no more new plants or meat is consumed. Over time, that portion of the elements in our bodies that is radioactive decays into a non-radioactive element. This means that if we can dig up the mastodon bones, and measure the fraction that is still radioactive, it will yield an approximate age. For example, a site containing bones showing high levels of a radioactive element would be younger than a site with bones showing only low levels of that same radioactive element.
In the case of the Cerutti Mastodon site, the bones and the bone material stuck on to the rocks thought to be used as tools by the early hominids, were measured to be 130,000 +/= 94,00 years old.
This technique of radiometric is the one most commonly used by archaeologists to measure the ages of interesting sites, and it is all thanks to cosmic rays from the Sun!