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Superkingdom Eukaryotae


acritarchs, histone proteins, ribosomes, archaea, chloroplasts

Eukaryotes evolved much later than prokaryotes, whose origins date to about 3.5 billion to 3.8 billion years before present. Alga-like fossils from ancient rocks suggest that eukaryotes may have evolved about 2.1 billion years before present. Other fossil remains indicate that eukaryotes were well established 1.6 billion years before present. These fossils, called acritarchs, are hollow spheres that appear to be spores or cysts of eukaryotic algae.

Eukaryotic cells are thought to have evolved from primitive prokaryotes. Evidence for this view is found in the archaea, prokaryotes that resemble both bacteria and eukaryotes. Like bacteria, the archaea lack a nucleus and most other organelles. Like eukaryotes, they display flexible cell membranes and histone proteins, and have certain segments of DNA in common. This evidence, along with other molecular studies, leads many scientists to conclude that archaea, bacteria, and eukaryotes arose from a common ancestral prokaryote similar to the archaea. However, according to a theory developed by American microbiologist Carl Woese, the archaea, bacteria, and eukaryotes may have arisen, not from a single common ancestor, but from a group of genetically diverse, primitive prokaryotes.

The nucleus and endoplasmic reticulum of eukaryotes probably evolved from internal folds of the plasma membrane of prokaryote ancestors. Scientists once thought that mitochondria and chloroplasts arose in the same way. But a theory proposed by American microbiologist Lynn Margulis holds that these organelles arose from certain prokaryotes, which established mutually beneficial relationships with the earliest eukaryotes. In the distant past, these free-living prokaryotes were consumed by early eukaryotes, but managed to survive within the cytoplasm. The autotrophic prokaryotes proved useful to their hosts because they produced glucose through photosynthesis. The heterotrophic prokaryotes also were useful because they could generate the energy source adenosine triphosphate (ATP) for the host. The host in turn provided protection for the engulfed prokaryotes. Over time, the autotrophic prokaryotes developed into chloroplasts and the heterotrophic prokaryotes developed into mitochondria. These organelles still contain their own DNA, with bacteria-like genes, and their own ribosomes, relics handed down from their distant bacterial ancestors.

Article key phrases:

acritarchs, histone proteins, ribosomes, archaea, chloroplasts, Eukaryotic cells, prokaryotes, mitochondria, cytoplasm, Eukaryotes, organelles, photosynthesis, spores, ATP, distant past, nucleus, relics, DNA, glucose, origins, time, hosts, theory, Evidence, turn, protection, group, scientists, way, years, view

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