All living things contain DNA and use the same four nitrogenous bases--adenine, thymine, cytosine and guanine in their genetic code to synthesize proteins needed for survival. As the environment changes, organisms must be able to adapt or they may perish. Molecules like DNA have evolved over time due to mutations. If we study present species' DNA sequences, we can learn about the past.
By comparing the genomes for various organisms, information can be provided about their evolutionary history throughout time in terms of genes that are shared and their gene products--proteins.
If a genome in one species and another are almost identical, that information can be used to piece together evolutionary history as to when these two species may have diverged from a common ancestor. The rate that a gene changes over time can be used as a molecular clock to figure out when two species diverged from an ancestral species. Specifically, the more similar the sequence of genes are in two species, the more closely related they are. If two species are compared and their gene sequences have many differences, it can be assumed they are more distantly related.
Even DNA fragments can be sequenced and studied from extinct organisms to show their relationships to modern organisms that still exist.
Sometimes, the human genome is compared to other species such as other Primates, to see what makes humans unique. In comparison to chimp DNA, the humans and chimps differ by slightly more than 1% in their DNA sequences. Some of these differences are evolving at a faster pace in humans and are genes involved in the immune system and genes regulating brain size.
In conclusion, comparative DNA studies can be used as evidence, along with the fossil record, comparative anatomy, comparative embryology, etc. to figure out Earth's evolutionary history.
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