WASHINGTON (AP) - Oxygen, key to life on Earth today, began to appear on the planet millions of years earlier than scientists had thought, new research indicates.

An analysis of a deep rock core from Australia indicates the presence of at least some oxygen 50 million to 100 million years before the great change when the life-giving element began rising to today's levels, according to two papers appearing in Friday's edition of the journal Science.

Previously, the earliest indications of oxygen had been from between 2.3 billion and 2.4 billion years ago when the "Great Oxidation Event" occurred.

The cause of the event is still not known, but before that the atmosphere was dominated by methane and ammonia. Today oxygen makes up about 21 percent of the atmosphere.

The discovery of traces of early oxygen was made in a study of a 3,000-foot-long rock core extracted in western Australia.

"We seem to have captured a piece of time before the Great Oxidation Event during which the amount of oxygen was actually changing - caught in the act, as it were," Ariel Anbar, an associate professor in Arizona State University's school of earth and space exploration, said in a statement.

The two research teams were led by Alan Jay Kaufman, associate professor of geochemistry at the University of Maryland and Anbar.

Carl Pilcher of the NASA Astrobiology Institute said: "Studying the dynamics that gave rise to the presence of oxygen in Earth's atmosphere deepens our appreciation of the complex interaction between biology and geochemistry. Their results support the idea that our planet and the life on it evolved together."