The endosymbiotic theory explains the origin of eukaryotic cells and how they got their chloroplasts and mitochondrion.
Based on theory, it is said that mitochondria and chloroplasts were originally single prokaryotic organisms that were engulfed by the eukaryote. (It is found that mitochondria and chloroplasts have their own DNA, so that is why it is thought that they were not originally a part of the eukaryotic cell.) Anyway, mitochondria and chloroplast soon developed inside of the eukaryote and became a necessity of the cell.
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For the first one, electrons travel around the nucleus in fixed energy levels with varying energies.
Second one, protons, neutrons, and electrons have a mass, but electrons have far less mass
Third one, cations and protons
Fourth, not sure.
He could perform some DNA tests that could show how those species were related. Genome sequencing could show all the differences in DNA nucleotides. The number of changes could then be multiplied by the rate of mutations and the result would show how species were related.