What trait of chloroplasts leads scientists to believe they were once independent microbes?

Why should we think that a mitochondrion used to be a free-living organism ... Based on decades of accumulated evidence, the scientific community ... The evidence suggests that these chloroplast organelles were also once free-living bacteria. ... to be the irreversible step at which the two independent organisms

there are mainly three domains in biological classification namely; archaea, bacteria, and eukarya.

archaea includes all the prokaryotic cells which are characterized by their lipid membranes. they have branched hydrocarbon which is attached to glycerol through an ether linkage.

bacteria includes all the prokaryotic cells, the cell membrane of which are made up of the unbranched fatty acid chain. in addition, they are attached to glycerol through an ester bond.

eukarya includes all the eukaryotic cells, that is, the cells contain a nucleus and other membrane-bound cell organelles.

they can be autotrophic as well as heterotrophic, unicellular as well as multicellular

they include kingdoms like protista, plantae, animalia, fungi et cetera.

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