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Early attempts at establishing an absolute time scale utilized the following concepts: declining sea levels, cooling of the Earth, cooling of the Sun, Earth tidal effects, sediment accumulation, and changes in ocean salinity.Using changes in Earth's temperature or the Sun's use of energy failed because energy from nuclear reactions was unknown.At the dawn of the twentieth century, physicists made a revolutionary discovery: elements are not eternal.

Some of these methods have helped to pin down the evolution of our hominid ancestors; anatomically modern humans evolved about 100,000 years ago.

While that's nearly 20 times older than the Earth was once thought to be, it's a geological eye blink.

Furthermore, this energy could penetrate cardboard and even walls to fluoresce barium platinocyanide.

Rontgen used the mathematical symbol for unknown to name this energy X-rays.

Darwin had argued that the Earth was immensely old — which gave his gradual process of evolution plenty of time to unfold.

The great physicist Lord Kelvin had countered that the planet was actually relatively young — perhaps 20 million years old.

Radioactivity also gave the history of life an absolute calendar.

By measuring the atoms produced by these breakdowns inside rocks, physicists were able to estimate their ages (right).

Mammals, which for 150 million years had been small, rodent-sized creatures, rapidly evolved to massive proportions in the wake of the Cretaceous-Tertiary extinction 65 million years ago.

Geological timekeeping continues to be a lively science, with new methods emerging all the time.

Nineteenth century geologists recognized that rocks formed slowly as mountains eroded and sediments settled on the ocean floor.