One major opposing viewpoint to the theory of evolution is the age of the planet Earth. According to most religious theories, the Earth is between four and ten thousand years old, created by an intelligent designer who also created the plant and animal life, fully formed. For a long time, there was no way to test the age of the Earth, as human records only go back to a certain point, and there is no definitive record otherwise. However, the science of radiometric dating created a scientific method of measuring the age of non-organic materials. Radioactive materials have a defined half-life, the amount of time it takes for that material to degrade by one-half; measuring the levels of radioactivity in rocks can show the likely age of that rock, and the age of the sedimentary layer that the rock is inside. Using this method, the age of the planet Earth has been estimated at around 4.6 billion years old, hypothetically more than enough time for single-cell organisms to evolve their biological functions, join into more complex organisms, and eventually give rise to larger animals. This age dating is supported by fossil records which are dated older than 10,000 years.