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comment Why our body does not produce polyunsaturated fatty acids?
I think the framing of this question is wrong. I would guess that our body doesn't produce PUFAs because they were present in adequate amounts in our ancestors' diets, so there was little pressure to gain the ability to synthesize them. The common ancestor of animals probably just didn't possess these enzymes, and we've evolved to make do with what's available in the diet.
comment How is evolution possible in contemporary humans?
To rephrase that paragraph in more PC language: some self-identified ethnic groups have higher birthrates than others. To the extent that allele frequencies in high-birthrate ethnic groups are different from the mean frequency for the population as a whole, the US population is "evolving" to increase the frequencies of those alleles. I added "race has a genetic component, but..." to specifically avoid an interpretation that genetics were the cause of increased birthrates.
comment How is evolution possible in contemporary humans?
I mean that there are differences in allele frequencies among the census bureau race/ethnicity categories. These are probably self-identified groups, but there are differences in allele frequencies between the group of people who self-identify as hispanic vs those who self-identify as white/caucasian. Alleles common in the former group are becoming more frequent relative to alleles common in the second group. Those would presumably include some alleles for skin pigmentation, among others (like variations in lactose tolerance, renal sodium channels, sickle-cell anemia, etc).