The origins of other aspects of eukaryotic cells are unclear.
Some researchers have proposed that the nucleus itself evolved from an endosymbiont.
Nuclear genes with close relatives in both bacteria and archaea have been found.
The genome of eukaryotic cells may be the product of genetic annealing, in which horizontal gene transfers occurred between many different bacterial and archaeal lineages.
These transfers may have taken place during the early evolution of life, or may have happened repeatedly until the present day.
The origin of other eukaryotic structures is also the subject of active research.
The Golgi apparatus and the endoplasmic reticulum may have originated from infoldings of the plasma membrane.
The cytoskeletal proteins actin and tubulin have been found in bacteria, where they are involved in pinching off bacterial cells during cell division.
These bacterial proteins may provide information about the origin of the eukaryotic cytoskeleton.
Some investigators have suggested that eukaryotic flagella and cilia evolved from symbiotic bacteria.
However, the 9+2 microtubule apparatus of eukaryotic flagella and cilia has not been found in any prokaryotes.
Concept 26.5 Multicellularity evolved several times in eukaryotes
A great range of eukaryotic unicellular forms evolved as the diversity of present-day “protists.”
Molecular clocks suggest that the common ancestor of multicellular eukaryotes lived 1.5 billion years ago.
The oldest known fossils of multicellular eukaryotes are 1.2 billion years old.
Recent fossil finds from China have produced a diversity of algae and animals from 570 million years ago, including beautifully preserved embryos.
Why were multicellular eukaryotes so limited in size, diversity, and distribution until the late Proterozoic?
Geologic evidence suggests that a severe ice age gripped Earth from 750 to 570 million years ago.
According to the snowball Earth hypothesis, life would have been confined to deep-sea vents and hot springs or those few locations where enough ice melted for sunlight to penetrate the surface waters of the sea.
Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 26-14