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The Origin of the Species' Cells

Call it evolution by acquisition. Billions of years ago, most scientists think, eukaryotic cells gained new talents by taking in bacteria. These became the miniature power stations known as mitochondria and chloroplasts. Researchers also think the nucleus, the genetic command center of the cell, arose when a pre-eukaryotic bacterium ate a so-called archaebacterium, but bacteria can't swallow other cells like eukaryotes can. Now, researchers report new molecular evidence that explains how all this ingestion was possible: The progenitor of the eukaryotic cell probably had an internal skeleton that allowed it to engulf other microbes.

The idea that a bacterium ate the more primitive archaebacterium to create the first eukaryote was recently bolstered. Researchers found that proteins in the nuclei of yeast most closely resemble archaebacterial proteins, and proteins in the yeast cytoplasm resemble bacterial proteins. However, the eukaryotic cytoplasm is far more complex than the inside of bacteria.

To determine where the eukaryotic cytoplasm came from, biochemist Hyman Hartman at the Massachusetts Institute of Technology and colleagues compared all the proteins of yeast with the proteins in the fruit fly, the worm, and a plant. The total number of common proteins reached about 2000. Then the group subtracted those that also occurred in prokaryotes, leaving about 900. Lastly, they removed proteins that were missing from one of the most ancestral eukaryotes, the parasite Giardia. The remaining 347 proteins are mainly associated with protein synthesis and the internal membranes that make up the cytoskeleton, the authors report in the 22 January issue of the Proceedings of the National Academy of Sciences. Hyman and his team suggest that the progenitor, which they dub the chronocyte after the Greek god Cronus who consumed his children, had properties unlike modern bacteria, such as the ability to engulf.

According to biochemist Russell Doolittle at the University of California, San Diego, this "systematic piece of work" by Hartman addresses an important event in evolution. Doolittle adds that although previous conclusions have been based on comparing a few proteins at a time, "nobody else has put the muscle to the job" like Hartman does in this study.

More on the chronocyte
Some background on evolution