When we eat protein, our bodies break it down into individual amino acids. These amino acids are then reassembled to make the specific proteins we actually need. So, when we think about eating “protein”, we should keep in mind that we’re really just gathering proteinogenic (“protein creating”) amino acids.
There are only 23 proteinogenic amino acids used by life, as we know it. Of these, 21 are present in eukaryotes: 20 are represented surjectively from codons, and the 21st (selenocysteine) is oddly encoded by the stop codon.
Of the 21 proteinogenic amino acids we use, 9 essential ones can’t be created by our bodies, and therefore need to be attained through food. Many foods are complete proteins, though, meaning they contain sufficient proportions of all essential amino acids. This is why thinking of “protein” rather than “amino acid” as a dietary element is usually sufficient.
Regarding the header picture of the pyramids above, the laborers of the pyramids consumed extreme quantities of meat:
In an area of the world where people have traditionally reserved meat eating mostly for special occasions and feast-days, we have found evidence that the ancient state provisioned the pyramid city with enough cattle, sheep, and goat to feed thousands of people prime cuts of meat for more than a generation—even if they ate it every day.
I learned about this from the amazing The Egyptian History Podcast.