One of the major criticisms of evolution has been centered on the incredible complexity of living things. Creationists argue that it is mathematically impossible for rare, chance events to have given us all the life forms we see on the planet. Evolution, relying on small random changes, requires vast amounts of time to work; however, five billion years, the estimated age of the Earth, is a vast amount of time. Scientist have created mathematical models that show that mutations occur at rates that make evolution possible.
Creationists have, however, attacked the scientific calculations of the Earth's age, saying it is much younger than scientists contend. A variety of radioisotope measures such as Carbon-14 dating confirm the scientific estimates, but creationists say that, since these methods depend on nuclear decay, they are only valid if the amount of cosmic radiation has not changed much over the millennia. Astronomers have studied other stars similar to our sun, and concluded that it is quite likely that cosmic radiation has not changed sufficiently to affect these dating methods.
Another common creationist criticism is the lack of intermediate forms in the fossil record. Such intermediates would show the gradual changes accumulated over generations, and would support the theory of evolution. The fact is that for some species, such as horses, we do have a good record of intermediate forms. For others, the lack of intermediates may be explained by the theory of punctuated equilibrium, which states that long periods of environmental and species stability may have been punctuated by episodes of instability, during which evolution occurred much more rapidly as challenging environmental conditions favored novel mutations rather than the status quo.