According to common-descent evolution, all living things on the planet evolved from one original living organism - by all accounts a unicellular, asexual, autotrophic, prokaryotic creature. How, then, did we get from that to multicellular, sexual, heterotrophic, eukaryotic creatures?
Evolution, of course. The first creature slowly evolved into colonial organisms with a diversity of trophic, reproductive, and genetic characteristics. Eukaryotic cell types came on the scene. Algaes and protozoans appeared. Primitive plants (like bryophytes and ferns) and primitive animals (like sponges and jellyfish) eventually evolved as well. After a few more million years, we have the diversity and complexity seen today among bacteria, protists, fungi, plants, and animals, including the advanced flowering plants and chordate animals.
So, the important question to be asked here is: how? What is the mechanism behind such remarkable change? There is one major source of new information: mutation. And there is one major driving force behind change: natural selection. While other types of evolution exist - such as speciation and adaptations - these two are the major players. How do they work?
When mutations accumulate in the genomes of some species, natural selection kills off the individuals that have "bad mutations" - i.e., mutations that make the organism weak. Thus, only good mutations are left and the species is left with a collective genome with more helpful information in it. Through this process, new species may arise and eventually (according to common-descent evolution), completely new and different types of organisms. For example, bacteria may become algae, seaweeds may become mosses, and dinosaurs may become birds.
Let's pause for a moment and consider mutations in our everyday lives. According to common-descent evolution, mutations were the original sources of information that allowed for the diversity we see today to stem from a single-celled prokaryote. Normally, mutations occur at a slow rate. But if you accelerate the mutation rate in a species, what happens to it?
It should just evolve faster. Right?
Set this up against what you know to be true in everyday life. What kinds of things can speed up mutation rates? Exposure to ultraviolet light, radiation from radioactive materials, and isolated population sizes.
Do you know why too much sun can lead to skin cancer? When ultraviolet light rays hit your skin cells, they damage the genetic material. In other words, they cause mutations to happen faster as your skin cells' DNA replicates. This accelerated mutation rate makes it more likely that one of your skin cells will get a genome with a specific mutation that tells the cell to stop replicating itself. Suddenly, you have a skin cell that starts dividing nonstop, and cancer has begun.
Radioactive elements work the same way. Their radiation accelerates mutation rates and increases your risk for cancer. In places like Chernobyl, radiation caused mutations in unborn babies that resulted in their being born without limbs. In the plant world, botanists can radiate seeds with radioactive rays to cause mutations in their genetic material. These mutations may result in variegation - leaves with green and yellow stripes. While this is pretty to people, the yellowing is caused by a lack of chloroplasts in that region of the leaf. The variegated leaves restrict the amount of photosynthesis possible, so this mutation would put it at a disadvantage in the wild.
Finally, consider what an isolated population size does to that population. Inbreeding in human populations can lead to physical deformities and mental problems. In one small mountain town in Kentucky, inbreeding resulted in a mutation that reduced the blood's capacity to carry oxygen (they had a hemoglobin deficiency). The people appeared somewhat blue as a result, and could not overexert themselves for lack of oxygen to the brain and muscles. All this occured because inbreeding accelerates mutation rates.
Dogs are another example of what happens when you limit the population size. The various breeds of dogs exist because breeders decided they like a certain trait and bred for it. Or perhaps, a mutation for short legs or mottled coats appeared which people liked. The breeders carefully selected parents that would carry these traits, and isolated the offspring from dogs who did not carry those traits. Over time, you have changed an original wolf-like dog to a Poodle, a Dalmatian, a Corgi, or a Shar Pei. It is well known that you risk purchasing a dog with physical illnesses - like epilepsy, bone problems, digestive or skin problems - if you purchase a pure-bred dog. If you get a "mongrel," or a mixed-breed dog of some sort, your chances of ending up with a genetically sick dog goes down drastically. Why is this?
It's because the mongrel comes from a pool of wider genetic diversity, and the pure-bred is the result of a selection for mutation after mutation, which invariably leads to a limited gene pool for the breed and a higher risk of defects.
Hopefully all this has made something very clear: mutations can only cause an organism to evolve downwards; it can only cause devolution. Natural selection has little real power here; it can only decide if the mutation increases the creature's chance of survival or decreases it. While some traits, such as the Greyhound's sleek figure and long legs, may increase its chance for survival in places where survival depends on speed, those same traits still represent an overall loss of genetic information. Put the Greyhound in a different environment - like a Russian winter, or a herd of cattle - and those same traits make it more vulnerable. Its thin structure makes it more susceptible to the cold, and its long legs make it harder to dodge a charging bull. The overall affect of the Greyhound's natural traits is a specialization that limits it to a specific environment, and a reduced gene pool from which to pull healthy genes.
The reason why mutations cannot result in common-descent evolution is the same reason why you can't marry your cousin. Mutations are overwhelmingly negative, and even if positive, they still result in a damaged gene pool. There have been extinctions in the wild simply because the gene pool got so small, the mutation rate became too high for the species to continue propagating itself. The cheetah is currently at risk of this type of extinction.
While a large population size can slow this from happening, the overall affect of mutations is still there. Mutations are still occurring, and they are still increasing the risk for genetic diseases and problems. We can slow this process, but we can't stop it. And we certainly can't reverse it.
In other words, we are evolving. We are evolving downwards. The human population is becoming a more and more defective breed of dog. According to Dr. John Sanford, the geneticist who invented the gene gun (which revolutionized the rapidly growing field of biotechnology), if humans have been evolving for the past two million years, we'd have gone extinct from mutation build up long ago. Under our current mutation rates, we would be expected to have a total of ninety million mutations since diverging from apes two million years ago. Since we only have ninety thousand mutations in the collective human genome so far, we are far from extinction.
Apropos, assuming humans began with a perfect genome, and have had relatively constant mutation rates based on current observations, humanity would reach 90,000 mutations after six thousand years. This is what we would expect if people are descendants of Adam and Eve, each of whom lived around 6,000 years ago. That kind of prediction is what good science is all about.
Anyway, the big takeaway is this: common-descent evolution is impossible because it is completely contrary to what we observe. We observe downwards-oriented evolution; common descent requires upwards-oriented evolution. You cannot get a duck from a t-rex with the evolution we observe today. And you cannot escape this fact with the ubiquitous claim, "but given enough time, anything is possible!"
Given enough time, we will all be a mess of helpless mutants, struggling with physical, mental, and genetic deformities. We are evolving towards extinction, not towards something bigger and better.
In other words, mutations bring a biological reason for the prayer, "Come, Lord Jesus" (Rev. 22:20).
Genetic Entropy and the Mystery of the Genome by John C. Sanford, PhD in Plant Genetics, professor at Cornell University
That book by Dr. Sanford is one of my most strongly recommended apologetics books.