Until the 1820s, hardly anyone even suspected that dinosaurs had ever existed. In the years since, paleontology has sought to discover the entire history of life on earth, from the era of single-celled organisms up into the human era. Paleontologists continue to make remarkable discoveries, such as that a huge meteorite that fell in the Gulf of Mexico wiped out the dinosaurs—all except the birds, the only surviving dinosaurs. "Radiometric dating" can reveal the age (often tens of millions of years) of a rock or fossil or a tiny grain of pollen by measuring how much its radioactive elements have disintegrated. The study of molecules of DNA, RNA, and proteins has also become important for dating. Paleontologists often consult with geologists searching for oil, gas, and coal deposits, since all these "fossil fuels" were formed from plant and animal remains.