Drawing on discoveries in biochemistry, astronomy, physics and related disciplines, the theory of intelligent design proposes that certain features of living systems and of the universe are best explained as the product of an intelligent cause, rather than purely undirected natural processes. Just like the "Big Bang" theory, intelligent design is a scientific theory that happens to have religious implications, but it does not depend upon religious premises. Because it has no basis in sacred texts, intelligent design is very different from and is not creationism.
The first scientific evidence supporting the theory of intelligent design is called "irreducible complexity." An irreducibly complex system is a single system composed of several well-matched, interacting parts that contribute to a basic function, and if you remove any one of these parts the system will not work. For example, a mousetrap cannot work unless the spring and all the other parts are in their proper location. Evolution says that over time life gradually became more complex in a step-by-step manner. However, Charles Darwin acknowledged, "If it could be demonstrated that any complex organism existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down." The bacterial flagellum, which is a motor-like structure at the end of some bacteria, is one such complex organism. Scientists have found that if any of the bacterial flagellum's parts are not in place, it will not function. Thus, it is one example of an irreducibly complex molecular machine, which could not have evolved in a step-by-step manner as envisioned by Darwin.
A second evidence for an intelligent cause of life is information-rich biological systems. Bill Gates has stated that, "Human DNA is like a computer program but far, far more advanced than any we've ever created." That's because the nucleotide molecules in a strand of DNA are specifically arranged in the precise sequences required for the formation of various proteins and, therefore, contain information. For instance, if someone threw a bunch of Scrabble letters up in the air, when they landed it would be impossible for them to form meaningful words. Just like specifically arranged letters in words contain information, so too the specifically arranged nucleotides in DNA contain information. Scientists arguing for the intelligent design of DNA do not do so merely because natural processes fail to explain the origin of this information in cells. Instead, they argue for design because they know from experience that systems possessing information invariably arise from intelligent causes. As the pioneering information theorist Henry Quastler observed, "Information habitually arises from conscious activity."
Thirdly, the fossil record shows that during the Cambrian geological period a great variety of new life forms appeared during a relatively short period of time. These new animal forms, grouped about some thirty different basic body plans ("phyla"), had large numbers of diverse cells with specific functions, including digestive, weapon, and locomotive systems. Scientists call this geologically sudden arrival of complex, different, and fully-formed animal body plans in the fossil record the "Cambrian Explosion." It's also known as "Biology's Big Bang" and is contrary to Darwin's "tree of life." The "tree of life" claims that initial life forms were similar and minimally complex and gradually branched out into more varied and complex life forms over long periods of time. However, the fossil record shows just the opposite. Namely, that during the Cambrian period many new and fully-formed animal forms appeared relatively suddenly and without any similar precursors.
Evolution states that more complex organisms gradually evolved over time from a common ancestor. If true, we would expect to find millions of transitional fossils in the fossil record, such as half fish - half frog. However, no clearly transitional fossils have been found. Evolutionists often say that erosion must have destroyed the transitional fossils. But, why did erosion "pick out" the transitional fossils and not the fossils that have been found? The lack of transitional fossils has not been adequately explained and is evidence against the theory of evolution.
The "fine-tuning" of the universe is additional evidence for intelligent design. Fine-tuning means that if the numerous constants and initial quantities in the laws of physics did not have the precise values that they do, then nothing like our habitable universe could exist. For example, if the expansion of the universe after the Big Bang had been only one millionth slower than it was, the universe would have collapsed back into itself. If it had expanded any faster, then no stars or galaxies would have formed. Also, if the gravitational force was stronger or weaker by an infinitesimal amount, then the sun and other life-sustaining stars could not exist. Another example is if the electromagnetic force were slightly stronger or weaker, life would be impossible. In addition, if neutrons were not exactly 1.001 times the mass of protons, protons would decay into neutrons or neutrons would decay into protons, and this would also make life impossible. Scientists have discovered over four dozen examples of fine-tuning in the universe and have calculated an infinitesimally small probability that these "cosmic coincidences" could have all occurred by chance. Thus, it is reasonable to conclude that an intelligent cause made it so.
In conclusion, scientific evidence such as irreducibly complex and information-rich biological systems, the "Cambrian Explosion," transitional gaps in the fossil record and the "fine-tuning" of the universe are some of the empirical challenges to the idea that chance and the unguided forces of nature alone caused the complexity of life on earth. The theory of intelligent design offers a compelling scientific explanation to the question of how we came to be; namely, that life has an intelligent cause.
For more information on Intelligent Design see: Signs of Intelligence by William A. Dembski and James M. Kushiner; The Case for a Creator by Lee Strobel; The Politically Incorrect Guide to Darwinism and Intelligent Design by Jonathan Wells;The Privileged Planet DVD by Illustra Media; and Unlocking the Mystery of Life DVD by Illustra Media.