At any particular time all living organisms have approximately the same ratio of carbon 12 to carbon 14 in their tissues.When an organism dies it ceases to replenish carbon in its tissues and the decay of carbon 14 to nitrogen 14 changes the ratio of carbon 12 to carbon 14.Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free.These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing.Thorium is not soluble in natural waters under conditions found at or near the surface of the earth, so materials grown in or from these waters do not usually contain thorium.In contrast, uranium is soluble to some extent in all natural waters, so any material that precipitates or is grown from such waters also contains trace uranium, typically at levels of between a few parts per billion and few parts per million by weight.By comparing fossils of different primate species, scientists can examine how features changed and how primates evolved through time.
The age of the fossil must be determined so it can be compared to other fossil species from the same time period.
At secular equilibrium, the number of thorium-230 decays per year within a sample is equal to the number of thorium-230 produced, which also equals the number of uranium-234 decays per year in the same sample.
Uranium-thorium dating has an upper age limit of somewhat over 500,000 years, defined by the half-life of thorium-230, the precision with which we can measure the thorium-230/uranium-234 ratio in a sample, and the accuracy to which we know the half-lives of thorium-230 and uranium-234.
Although most attention in today's world focuses on dinosaurs and why they became extinct, the world of paleontology includes many other interesting organisms which tell us about Earth's past history.
The study of fossils and the exploration of what they tell scientists about past climates and environments on Earth can be an interesting study for students of all ages.