What gave rise to the enormous diversity of protists that exist today? There is abundant evidence that much of protist diversity has its origins in endosymbiosis, the process in which certain unicellular organisms engulf other cells, which become endosymbionts and ultimately organelles in the host cell. For example, as we discussed in Chapter 25, structural, biochemical, and DNA sequence data indicate that the first eukaryotes acquired mitochondria by engulfing an aerobic prokaryote (specifically, an alpha proteobacterium). The early origin of mitochondria is supported by the fact that all eukaryotes studied so far have either mitochondria or modified versions of them.
Much evidence also indicates that later in eukaryotic history,
a lineage of heterotrophic eukaryotes acquired an additional
endosymbiont—a photosynthetic cyanobacterium—that then
evolved into plastids. As the hypothesis illustrated in
Figure 28.2 shows, this plastid-bearing lineage gave rise to two
lineages of photosynthetic protists, or algae: red algae and
green algae. This hypothesis is supported by the observation
that the DNA of plastid genes in red algae and green algae
closely resembles the DNA of cyanobacteria. In addition, plastids in red algae and green algae are surrounded by two membranes. Transport proteins in these membranes are homologous
to proteins in the inner and outer membranes of cyanobacterial
endosymbionts, providing further support for the hypothesis.
On several occasions during eukaryotic evolution, red algae
and green algae underwent secondary endosymbiosis: They
were ingested in the food vacuoles of heterotrophic eukaryotes
and became endosymbionts themselves. For example, protists
known as chlorarachniophytes likely evolved when a heterotrophic eukaryote engulfed a green alga. Evidence for this
process can be found within the engulfed cell, which contains a
tiny vestigial nucleus, called a nucleomorph. Genes from the nucleomorph are still transcribed, and their DNA sequences indicate the engulfed cell was a green alga. Also consistent with the
hypothesis that chlorarachniophytes evolved from a eukaryote
that engulfed another eukaryote, their plastids are surrounded
by four membranes. The two inner membranes originated as
the inner and outer membranes of the ancient cyanobacterium.
The third membrane is derived from the engulfed alga’s plasma
membrane, and the outermost membrane is derived from the
heterotrophic eukaryote’s food vacuole. In some other protists,
plastids acquired by secondary endosymbiosis are surrounded
by three membranes, indicating that one of the original four
membranes was lost during the course of evolution.
