Red Mars contains many examples of science altering nature. Since we're a part of nature, we guess that means human beings are significantly altered by these futuristic technologies as well. But it's not by radioactive spiders or freak gamma ray accidents. These genes are altered on purpose.
With filigree pockets and everything. Okay, bad joke, but actually not far from the truth. Just like designers stitching together the perfect pair of denim, the scientists on Mars learn to stitch together human DNA, designing new humans as a result.
The most prominent example of this in the novel is the gerontological treatment. Pioneered by Vlad and Ursula, this genetic therapy strengthens DNA by an "auto-repair genomic library" that "will replace the broken strands" of DNA (5.6.25). While it doesn't prevent death, it does significantly prolong the life of anybody who takes the treatment. While we don't want to go into spoiler territory, it's only the first of many such DNA-altering treatments to be incorporated into the Mars trilogy.
The symbolism is pretty obvious here: science and its advancements will make human life significantly better. Then again, Robinson isn't the type of author to give us the easy answer, now is he?
The treatment will strengthen humanity and the human genome. As Arkady points out, it gives human beings the ability to look at the world and think, "Why not make this [place] more rational?" (5.9.10). But this ideal comes with the warning that it can only happen if society is ready for it and has the room for the scientific advancement to actually help.
Not sure what we're talking about? Consider this: when the longevity therapy is made known to the wider world, it causes distress. If the treatment only goes to the rich, then the poor will revolt—but if it goes to everyone, overpopulation will get the better of Earth (5.9.102). Either way, humanity is not ready for it, but there it is all the same.
The Heart of the Matter
Genetic imagery expands beyond just humans, too. When John visits Phobos, he looks down at Mars and:
Suddenly it looked to John like a great orange cell, or embryo, or egg. Chromosomes whipping about under a mottled orange shell. A new creature waiting to be born, genetically engineered for sure; and they were the engineers, still working on what kind of creature it would be. (5.9.43)
In rewriting human genes, Martian scientists are attempting to make humanity better. But as this passage suggests, the environment is just as important in the cause of bettering ourselves. The Mars colonists have the power to alter both, but the question is: what will they create with their powers? In what we call the Spock choice, will they help the many or the few?
Although the answers may be up in the air, we can say this at least: in Red Mars, genes and environment are equally important in determining who we are and what we will become.