Saturday Science: Earth: A Biography: Eukaryotes, or How I Developed a Nucleus and Changed the World

We still in the Precambian era, but we've progressed to about three billion years ago, the earth's tectonic plates have formed and have started moving around, cyanobacteria have started releasing oxygen, which is going to take a while to have an effect on the types of life found on earth (like about a billion years), but it will, and we have archea, bacteria, and eukaryotes.

There's a fair amount of controversy about the relationships between these three groups. The article on the Archea at Wikipedia is fairly detailed and quite informative (and heavily referenced, so I'm taking it as fairly accurate -- it's not like it's the entry on Donald Trump or something), and has this to say about the relationships among the archea, bacteria, and eukaryotes:

The evolutionary relationship between archaea and eukaryotes remains unclear. Aside from the similarities in cell structure and function that are discussed below, many genetic trees group the two.

Complicating factors include claims that the relationship between eukaryotes and the archaeal phylum Crenarchaeota is closer than the relationship between the Euryarchaeota and the phylum Crenarchaeota[69] and the presence of archaea-like genes in certain bacteria, such as Thermotoga maritima, from horizontal gene transfer. The standard hypothesis states that the ancestor of the eukaryotes diverged early from the Archaea, and that eukaryotes arose through fusion of an archaean and eubacterium, which became the nucleus and cytoplasm; this explains various genetic similarities but runs into difficulties explaining cell structure.[73] An alternative hypothesis, the eocyte hypothesis, posits that Eukaryota emerged relatively late from the Archaea.

A recently discovered lineage of archaea, Lokiarchaeum, named for a hydrothermal vent called Loki's Castle in the Arctic Ocean, has been found to be most closely related to eukaryotes. It has been called a transitional organism between prokaryotes and eukaryotes.

(Citations removed.)

Phylogenetic tree showing the relationship between the Archaea and other domains of life. Eukaryotes are colored red, archaea green and bacteria blue. Adapted from Ciccarelli et al. (2006)

Now, this all comes with a big "but" -- whatever the eukaryotes developed from, we still don't know how it happened. Given that we're dealing with single-celled organisms, fossil remains are unlikely. Remember, the evidence that we do have for early life forms isn't comprised of actual fossils of the organisms themselves, but of "tracers" that they left behind -- graphite in zircons and fossilized stromatolites. So we can't point to a fossil of something and say "See? That's the missing link between the archea and the eukarya." So we have to infer a lot.

This article is a little dense in places, but gives a good idea of some of the important differences between prokaryotes (archea and bacteria) and eukaryotes:

There is a sharp divide in the organizational complexity of the cell between eukaryotes, which have complex intracellular compartmentalization, and even the most sophisticated prokaryotes (archaea and bacteria), which do not. A typical eukaryotic cell is about 1,000-fold bigger by volume than a typical bacterium or archaeon, and functions under different physical principles: free diffusion has little role in eukaryotic cells, but is crucial in prokaryotes. The compartmentalization of eukaryotic cells is supported by an elaborate endomembrane system and by the actin-tubulin-based cytoskeleton. There are no direct counterparts of these organelles in archaea or bacteria. The other hallmark of the eukaryotic cell is the presence of mitochondria, which have a central role in energy transformation and perform many additional roles in eukaryotic cells, such as in signaling and cell death.

The conservation of the major features of cellular organization and the existence of a large set of genes that are conserved across eukaryotes leave no doubt that all extant eukaryotic forms evolved from a last eukaryote common ancestor (LECA; see below). All eukaryotes that have been studied in sufficient detail possess either mitochondria or organelles derived from mitochondria, so it is thought that LECA already possessed mitochondria (see below). Plants and many unicellular eukaryotes also have another type of organelle, plastids.

(Citations removed.)

So you can see that we're dealing with a quantum leap in complexity and functionality. And note also that mitochodria, which play such an essential role in cell metabolism, also, according to some theories, represent what we can only describe as a symbiotic relationship:

The endosymbiotic hypothesis for the origin of mitochondria (and chloroplasts) suggests that mitochondria are descended from specialized bacteria (probably purple nonsulfur bacteria) that somehow survived endocytosis by another species of prokaryote or some other cell type, and became incorporated into the cytoplasm. The ability of symbiont bacteria to conduct cellular respiration in host cells that relied on glycosis and fermentation would have provided a considerable evolutionary advantage. Similarly, host cells with symbiont bacteria capable of photosynthesis would also have an advantage. In both cases, the number of environments in which the cells could survive would have been greatly expanded.

Here's a nice comparison of your basic, generic prokaryotic cell and your basic, generic eukaryotic cell:

You can see that eukaryotes, even the single-celled variety, are much more complex than their forebears. They also, in evolutionary terms, have a huge advantage, being able to adapt to a greater range of environments. Granted, there aren't nearly as many environments on Earth as there will be later, but eukaryotes were able to adapt to more of them -- a trait that will continue.

We're going to take a leap in time for next time because for the next couple billion years, not much was happening. Except sex. Brace yourself.