Now researchers have found a way to date ancient sediments that could considerably shrink the uncertainty and clarify such early Earth phenomena as the evolution of animals. Sedimentary rocks are hard to date because they're made of particles from older rocks. And Precambrian rocks contain few if any animal fossils. Younger sedimentary rocks, which hold about 12 percent of Earth's history, contain abundant fossils that are accurate age indicators. Geologists from the University of Western Australia solved the Precambrian puzzle by analyzing the mineral xenotime, which crystallizes in the small spaces between grains in sedimentary rock shortly after deposition.
Xenotime contains uranium, which slowly decays to lead.
After using a beam of charged particles to free lead from tiny xenotime crystals, researcher Neal McNaughton and colleagues measured the lead's composition and used it to calculate age. If xenotime dating works as well for other rocks, it could markedly improve understanding of the evolution of animals and other key Precambrian events. Exactly when the first animals evolved is a topic of hot debate, with estimates ranging from 1.2 billion to less than 600 million years ago. According to Jere Lipps, a paleobiologist at the University of California-Berkeley Museum of Paleontology, narrower age ranges would also help explain why animals appeared by clarifying how evolution relates to environmental events such as Ice Ages and continental drift, which leave traces in the rock. Samuel Bowring, a geologist at the Massachusetts Institute of Technology, cautions that the accuracy of a xenotime date depends on how soon the mineral forms, and that it may take much more than 1 million years.