A eukaryote is a life form whose cells have organelles wrapped in a plasma membrane. Eukaryotes are that small proportion of life on Earth that aren't bacteria or archaea: in other words, eukaryotes are the smallest of the three domains of life. You, dear reader, are probably a eukaryote.
It's generally a good idea to understand the eukaryotic cell and its evolution if one plans to get into a debate, because one of the better educated arguments against evolution appears to be the difficulty in developing membrane-bound organelles in the first place. After all, bacteria get along just fine without 'em. Of course, this has been explained for decades, but that never stops a good Creationist from arguing it as though it's a stumper.
 Primer on Organelles
There are a wide number of membrane-bound organelles that can be found in eukaryotes, but the real money-maker is the nucleus. Nuclear membranes are typically double membranes; most other organelles only get one membrane. Inside the nucleus is the DNA of the cell, whose primary role is providing badly written science mumbo jumbo for Hollywood movies. In most or all eukaryotes, there are multiple chromosomes inside the nucleus, each containing a single linear DNA strand. By comparison, prokaryotes (bacteria and archaea) have circular DNA. Cool, eh?
The other key organelle to know about is the mitochondrion, plural mitochondria. Mitochondria are very common amongst eukaryotes, but not universal. If you plan to argue about evolution on a cellular scale, you must understand mitochondria. First of all they're cool, and second they're the one of the biggest aces in the hole we have ever discovered in favour of evolution on this particular microscopic level. Anyway, from a cellular standpoint mitochondria are responsible for supplying energy and stuff.
There are plenty of other organelles in eukaryotic organisms, but here's just a short list. Unless you're really into cellular biology, you're not likely to need to know this stuff in the average debate with a crackpot.
- Ribosomes - worth mentioning because these guys are not membrane-bound, and can therefore be found in prokaryotes as well! Oh snap!
- Endoplasmic reticulum and Golgi - interesting in that these organelles are pretty much entirely composed of the plasma membrane in question
- Vacuoles and vesicles - these are just little pockets of membrane holding stuff. There are many types, defined by the stuff they hold. My favourite are lysosomes because they sound cool and hold deadly acid.
 Evolution of eukaryotes
Haha, we're being silly. Of course eukaryotes didn't evolve, they arose from the mind of an intelligent designer who, for some reason, made a clear pathway of development from prokaryotes to eukaryotes all at once just to confuse us once we invented microscopes. That rascal!
 Endosymbiotic theory
The most elementary theory for organelle development, in terms of easiness-of-understanding, is the endosymbiotic theory. Remember in the previous section how I said you had to understand mitochondria? I'm going to assume you do now, so if you don't I take no responsibility for what happens.
Basically the endosymbiotic theory hinges on two nifty things.
- Mitochondria are eerily similar to prokaryotes in terms of their DNA structure, replication mechanisms, protein structure and, well, pretty much everything. They're pretty closely related (as far as millions of years of separate evolution can allow) to the Rickettsia genus, in fact.
- The few eukaryotes which lack mitochondria are the most primitive. This isn't because they have poor table manners and still struggle with understanding the wheel; rather, it's because we can trace back the the ribosomal RNA - the sequence of nucleic acids that makes up their ribosome organelles. This stuff can't change much because if it does, it usually results in a complete replication error and cellular death. Thus, rRNA is a great way to track changes over the long term. Since ribosomes, regardless of rRNA structure, all work pretty well, the only reason an intelligent designer would do this would be to try to deceive us into believing in evolution. The eukaryotes which lack mitochondria diverged from the main line a very, very long time ago. This fits perfectly with a single-evolutionary-event model as in endosymbiotic theory.
Basically, endosymbiotic theory states that at some point in the early development of eukaryotic organisms, some amoeba or other happened to swallow a rickettsia-like bacterium, but failed to digest it properly. That's not actually that unlikely: modern rickettsia are obligate intracellular parasites already, or in English, they can only live inside a cell where, like children, they eat all the food and give nothing back. So the amoeba winds up with this little bastard living inside it, eating its food and wasting its time. The trouble is, the bacterium in question also suffers: it has to live inside the amoeba, that's what obligate parasite means, but it's making the amoeba weak, essentially lowering the property value of its house just by living in it. Somewhere along the line the endoparasite developed a mutation that allowed it to take some of the work away from the amoeba's general work-of-living, and suddenly that endoparasite-infested amoeba started growing way faster than its peers. Thus, symbiosis was achieved. In fact, protomitochondria-containing amoebae became so much better than their peers that only three lines of non-mitochondrial eukaryotes remain.
 common debate 'gotchas'
 scientific response to said 'gotchas'
 Neat stuff to know at parties
Water bears are cool and yeast can use plasmids.
- ↑ Cellular "organs" analogous to the organs in a human being, but smaller
- ↑ Alternatively you might be a software program, or possibly a climate change denialist
- ↑ This is very, very relative of course
- ↑ Dear Creationists: if you want to be difficult, learn a lot about prebiotic evolution. It's not that we don't have good theoretical answers for questions about prebiotic evolution, it's just that it's a way more interesting debate.
- ↑ This isn't quite the same as prophylaxis on those special long nights, mainly because the nuclear membrane is studded with pores
- ↑ Except the mitochondrion.
- ↑ All, to my knowledge, but I'm not giving up on there being some crazy yeast out there to prove me wrong
- ↑ You'll often hear people use "mitochondria" to refer to one mitochondrion. If you do, Rule 92 of Science states that you must fill the speaker's mouth with Cool Whip. This error is so common that my spell check doesn't even recognise the singular form.
- ↑ If you like plants, you may be interested to know that mitochondria are closely related to chloroplasts too. Chloroplasts are neat I guess, they turn light, water, and air into sugar. If you're into that kinda thing.
- ↑ Just like in James Bond movies.
- ↑ But they do, oh they do
- ↑ This isn't as uncommon as it sounds. Viruses are also obligate intracellular parasites. Heck, so is chlamydia, so you probably have an intracellular parasite if you were brought up with abstinence education
- ↑ Hey Tyrannis, u mad bro?