Image: RYAN REID These days hardly a week goes by without
important discoveries concerning the history of life on Earth
making headlines. Indeed, just last month researchers described
a fossil that pushes the origins of key mammal features back
some 45 million years. And last week scientists announced that
new dates for an extinction event that claimed most of
Australia's large animals show that humans, not the climate,
wiped them out. Although visual inspection of the rocks, fossils
and archaeological remains used to reconstruct our planet's past
provides critical information, only by ascertaining their ages
can researchers put this data into a meaningful context. The
first step toward accurately measuring geologic time came at the
turn of the 20th century, when French physicist Henry Becquerel
discovered the natural radioactive decay of uranium. Shortly
thereafter, building on related work by Ernest Rutherford,
American chemist Bertram Borden Boltwood determined that he
could use the predictable decay of ra