From Charles Darwin to Francis Crick, arguments as to the age of the Earth have had a large influence on the development of biology as a science. Before there could be a theory of evolution, there had to be a dramatic expansion in the perceived age of the planet.
"The poor world is almost six thousand years old," said Rosalind to Orlando, in Shakespeare's As You Like It, and she went on to explain that in all that time no man had ever died for lack of love. Implicit in her remarks was the perception that 6,000 years was A VERY LONG TIME, and in the popular mind, it certainly is, relative to an individual's lifetime. Rosalind's estimate was based essentially on a calculation of the "begats" in the bible, adding up the lifespans of the generations between Adam and Jesus.
The first to appreciate the antiquity of the Earth appears to have been James Hutton, a Scottish natural philosopher. Based on observations of uplift, sedimentation and erosion, his Theory of the Earth, published in 1788, concluded that the age of the earth was essentially infinite with "no vestige of a beginning—and no prospect of an end." promptly got Hutton denounced as an atheist. Nevertheless, Charles Lyell extended and popularized his work with the publication of his own Principles of Geology in 1830. He maintained that the measurement of processes like erosion in the present era could be extended backwards to give some idea of the time required to create great geological features like river gorges. From the lack of fossil remains in older strata, Lyell concluded that man was of recent origin on the Earth.
Darwin carried a copy of Lyell with him on his famous voyage on the Beagle. With the expanded time horizon afforded by geology, Darwin was able to see how various species might arise by natural selection from a common precursor.
From radiological dating of meteorites and homogenous terrestrial deposits of lead, it has been estimated that the Earth is about 4.6 billion years old. Recent results from Lars Borg at Lawrence Livermore National Laboratory, obtained by isotopic tests of lead, samarium, and neodymium in lunar rocks suggest that at the moon is younger than that—a mere 4.36 billion years. Since the moon is thought to have formed from material bounced out of the proto-Earth by collision with a massive object, this might imply that the Earth is actually younger than we thought. In any case, the oldest fossilized remains of living organisms are about 3.4 billion years old, giving a minimum estimate for the span of time evolution has been progressing--practically from the time the planet was cool enough for life to exist.
Ironically, 3.4 billion years was not enough for Francis Crick and Salk Institute colleague Leslie Orgel. They argued that there was insufficient time for life to have been generated abiotically and then to have evolved to its current state. The first steps are the most difficult. As biologist Lynn Margulis pointed out, "To go from a bacterium to people is less of a step than to go from a mixture of amino acids to a bacterium." Therefore, according to Crick and Orgel, never strangers to controversy, life must have gotten a head start somewhere else, to be seeded on Earth from outer space, an idea called panspermia.