Firstly, our genes can experience chance mutations which alter the blueprints that instruct our bodies how to grow and develop into adulthood
These mutations can then be passed from parents to offspring and enter the gene pool. If these mutations cause an organisms to be better adapted for their
current environment then the organisms will be able to reproduce more successfully before they die. If an adaptation puts an organism at a disadvantage (prom prey or other environmental factors) then that organism may die before
it had the chance to reproduce. This is known as “survival of the fittest” and it’s how natural selection works!
If a species becomes isolated from the rest of its group (by a mountain chain forming, or through habitat loss for example) any mutations that occur and are retained in the isolated population
will not enter the rest of the population through sexual reproduction- this is often how speciation occurs! So new species with different adaptations arsing from mutations can form over time.
The biodiversity we see today, having stemmed from single-celled organisms has arisen through small incremental steps like I’ve described over about 2-3 billion years! It’s not hard to imagine
when you take those sorts of time frames into account!
Interesting question. Essentially, all changes have come about through mutations at the genetic level. So, and increase in complexity (either in terms of the amount of genetic material, or what it codes for), has arisen as a random mutation, which has offered an advantage and so been selected for. It is hard to visualise this, but logically it makes sense. The speed and direction of change simply depends on what challenges the environment is presenting to the organism.