The Big Bang Creation of EukaryotesM.70
The most visible difference between prokaryotes and eukaryotes is the existence of a nucleus which holds the cell's DNA (see figure). This is only one of many important differences, some of which are mentioned below.
The sudden appearance of eukaryotes at about 1.8 Ga (Billion years ago), with no clear predecessors and with major structural and genetic changes, has been described as a "Biological Big Bang". Not only do the eukaryotes appear on the scene suddenly and without warning or obvious predecessors, but even within the major types of eukaryotes there is no clear line of antecedence—no clear "archeotype". Indeed the first eukaryote appears to be an example of "the magnates walk first"M.73—the (unknown) ancestor is more complex than any of the descendents. This is a recurring theme in the development of species.M.74
The first eukaryotic cells were single-celled (the protists), as bacteria are. Multicellular structures formed by cyanobacteria are actually individual single-celled microbes that live attached, and may have some cell specialization such as the nitrogen-fixing heterocysts and akinetes. But the structural and transport features of the proper cell have the potential for far more, and thus led in time to multi-celled species, and eventually to the visible, multi-cellular plants and animals.
There are six major groups of eukaryotes (see Wikipedia), and the inferred "common ancestor" must have been more complex than any existing (very complex) group. Many traits of eukaryotes are unique to them and have no analog in bacteria.M.75
• Eukaryotes are a radical advance in organizational complexity over all prokaryotes.
• All eukaryotes display a large package of genes that are unknown—let alone separately existing—in any prokaryotes.
• The genes that do appear to be shared with prokaryotes seem to be indiscriminately selected from both archaea and bacteriaM.76. There is no plausible scenario in which this might occur by natural evolution—even given lateral gene transfer, which is well-established, demonstrated in the laboratory, and can occur between unrelated species.
The following table lists some of the Eukaryote innovations. The mitochondria are the "powerhouse of the cell"—main producers of ATP, and use oxygen to do this: this is the reason why all eukaryotes require oxygen (even plants—derived from photosynthesis).
The invention of the eukaryotic cell came at a critical time in the history of the Earth, at a time when most of the reduced minerals in the oceans and Earth crust had oxidized and the atmosphere's oxygen content had reached a level that could poison cyanobacteria.
The invention of the original eukaryote could be another example of starting with a "magnate" (see above). With the advent of detailed gene studies, one might have hoped that the problem of complexity of the first "proto-eukaryotes" might be solved, but such is not the case. Consider the following statement regarding the identity of the very earliest eukaryotes, summarizing a decade of intensive genetic research on the origin of the six major groups of eukaryotes:
"There are therefore no grounds to consider any group of eukaryotes primitive... Rather it is becoming increasingly clear that most or perhaps all of them evolved from more complex ancestral forms by reductive evolution. Reductive evolution refers to the evolutionary modality typical of parasites: they tend to lose genes, organelles and functions when the respective functionalities are taken over by the host. So the archezoan (crown group) phylogeny seems to have been disproved, and deep phylogeny and the theories of the origin of eukaryotes effectively had to start from scratch."M.79