Once or Many Times
One of the biggest questions in biology regards the development of the eucaryote cell. Procaryotes(bacteria and archaea) are relatively simple cells with no sharply defined structures. The genetic material tends to take the form of rings that move within the cytoplasm of the cell, and there is relatively little in the way of cellular machinery, particularly structures associated with energy and metabolism. As a result, procaryotic cells tend to be small.
By contrast, eucaryotes have a distinct nucleus containing their genetic material, which is typically arranged in chromosomes, as well as distinct, specialized structures (organelles) in the cytoplasm to carry out various cellular functions, enabling them to be much larger and more energetic than procaryotes, and ultimately form multicellular organisms. One of the most important of these organelles is the mitochondrion, which has a central role in metabolism. It also has its own genetic material, independent of the genetic material in the nucleus -- and because this genetic material is only transmitted via the maternal line, it is critical in understanding the migrations of populations -- and is also used to identify remains by comparing them to mitochondrial DNA from relatives on the maternal line.
That DNA is also very similar to the DNA of some bacteria, which has led evolutionary biologists to conclude that the ancestors of mitochondria were free-living bacteria which were at some point engulfed by some other procaryote but not digested, instead entering a form of symbiosis and over many generations becoming entirely dependent upon the host cells. However, there was still the question of how the host cell came to have a distinct nucleus and other organelles. Did they come first as part of a slow stepwise development, at which point mitochondria were acquired, or did the mitochondria come first. Early efforts were inconclusive, even when the archaea were recognized as a sister clade of the bacteria. Eucaryote genes seemed to have striking resemblances to both clades.
Now a new method of analysis is pointing to a single origin of eucaryotic life -- that the merger of bacterium and archaean to form a eucaryotic cell was a unique event, never replicated in the history of life on Earth. Therefore, all multicellular organisms in all their diversity are descended from this one extraordinary moment when an archaean swallowed a bacterium but, rather than digesting it, incorporated it into its own systems.
It may also point to a solution to Fermi's Paradox -- that the Great Filter is already long past us, and when we go out there, we may well find enormous numbers of worlds inhabited by bacteria, but we alone account for all the complex life in the universe.
By contrast, eucaryotes have a distinct nucleus containing their genetic material, which is typically arranged in chromosomes, as well as distinct, specialized structures (organelles) in the cytoplasm to carry out various cellular functions, enabling them to be much larger and more energetic than procaryotes, and ultimately form multicellular organisms. One of the most important of these organelles is the mitochondrion, which has a central role in metabolism. It also has its own genetic material, independent of the genetic material in the nucleus -- and because this genetic material is only transmitted via the maternal line, it is critical in understanding the migrations of populations -- and is also used to identify remains by comparing them to mitochondrial DNA from relatives on the maternal line.
That DNA is also very similar to the DNA of some bacteria, which has led evolutionary biologists to conclude that the ancestors of mitochondria were free-living bacteria which were at some point engulfed by some other procaryote but not digested, instead entering a form of symbiosis and over many generations becoming entirely dependent upon the host cells. However, there was still the question of how the host cell came to have a distinct nucleus and other organelles. Did they come first as part of a slow stepwise development, at which point mitochondria were acquired, or did the mitochondria come first. Early efforts were inconclusive, even when the archaea were recognized as a sister clade of the bacteria. Eucaryote genes seemed to have striking resemblances to both clades.
Now a new method of analysis is pointing to a single origin of eucaryotic life -- that the merger of bacterium and archaean to form a eucaryotic cell was a unique event, never replicated in the history of life on Earth. Therefore, all multicellular organisms in all their diversity are descended from this one extraordinary moment when an archaean swallowed a bacterium but, rather than digesting it, incorporated it into its own systems.
It may also point to a solution to Fermi's Paradox -- that the Great Filter is already long past us, and when we go out there, we may well find enormous numbers of worlds inhabited by bacteria, but we alone account for all the complex life in the universe.