1) How did the first, early single celled organisims make the transtion from plants to animals? Did plants and animals have different starting cells, or did the cell(s) branch off?

Excellent question. Plants and animal cells are actually extremely similar. They belong to one of three domains of different cell families. The first domain is Eubacteria. These are the common bacteria found everywhere on the planet. They are by far the most permeating life form in existence, and make up 90% of the dry biomass of the Earth. The second is the less common, more elusive Archaeabacteria, which tend to hide in the lithosphere surviving on an anaerobic cocktail of nutrients, but can also be found in bogs and lakes, and even our own stomachs.

The third and most familiar is the Eukaryota. These are what make up plant and animal cells. They are large and complex, almost 1000 times the size of the average bacteria. Each Eukaryotic cell is like a small city with lots of organelles swimming in a giant cytoplasm. Don't believe me? Check out that video on youtube called the inner life of the cell.

Eukaryotic cells can be classed as either plants or animals or fungi. Ancient Eukaryota originated as predators. They had a fluid highly flexible cell membrane that allowed them to envelope and capture nutrients. This function is retained in some cells like T-Cell lymphocytes, Leukocytes, and other cells in the immune system. This fluidity also allowed them to capture small bacteria. This is believed to be how our organelles in the cells came to be. There is a huge amount of compelling evidence that mitochondria, lyosomes, peroxisomes etc all the organelles inside the cells, are ancient bacteria that have since evolved into a symbiosis with the Eukaryotes.

So the ancient Eukaroytes were predators, trundling around and engulfing food. To look at how plant cells diverged from this, we need to understand a tiny little bacteria family called cyanobacteria.

These little bacteria may have been the first life forms in existence. They are known to have existed for almost 3.8 billion years. They perform a process called photohydrolysis. Sound familiar? It is a precursor to photosynthesis. These little organisms were the first photosynthetic organisms. Indeed, they were responsible for creating todays oxygen-based atmosphere. 

recall that the Primordial Eukaryotes would simply trundle along engulfing hapless prey? The plants were ancient Eukaryotes that swallowed the cyanobacteria. They became incorporated into the Eukaryotic machinery, allowing it to do what the bacteria does. Which is to obtain glucose by photohydrolysis.

And if the cells can do that...there is no need to chase after prey anymore. The plant cells were revolutionaries. They made the transition from hunting to farming around two billion years ago. So they lost their fluid bilayer (dont need it anymore) replacing it with a rigid cellulose wall which was useful for stacking chloroplast cells. They lost their ability to engulf. A process which by the way is called (this is my word of the day) phagocytosis.

It seems to have worked out OK for them... 

2) Which came first, plants or animals or did they come at the same time? 

Plants. Definitely. The original Eukaryotic cells still make up animal cells, however, those cells when they exist indepedently or in simple colonies are called protozoa. When plant cells clump to form algae, it can legitemitely be called a plant. For an animal to be an animal however, it requires blood vessels, neurons etc. The first animal was a sea sponge. It came almost a billion years after the first plant.

 3) Is plant evolution different from animal evolution? How?

Not really. Same processes apply. Divergence. Duplication. Homology. Natural selection. There are 300,000 odd species of plant. They are all extremely similar. 

No. Only those who cannot do teach. Just kidding.

Wow. From one little bacteria strain to the tallest tree. Amazing.

Thanks. 

I thought that the first primitive cells were called Prokaryokes, and the cells that you mentioned, the more advanced ones Eukaryokes?

Yes thats correct. I wasnt talking about the origin of cells. That was not his question. I was talking about the divergence of plant cells from ancestral Eukaryotes

I agree with your response to the first point, but here I have a few caveates.

 It's been a while, but IIRC, all multi-cellular plants plants post-date the Cambrian. Until then we only had single celled protists like algae, while there were true muticellular animals before there were muti-cellular plants.

Still, like you said, it is obvious that eukaryotes postdate prokariotic cells.

Prokaryotes are the more primitive of the two and it is rightly believed that eukaryotes evolved from them. The first eukaryote was likely a colony of prokaryotes that became incorporated.

Organelles, like mitochondria and chloroplasts, are the internal components of the cells, they are similar in function to your body's organs - they carry out chemical and catalyic reactions.

As an example, the mitochondria and chloroplasts of modern eukaryotes have features similar to whole prokariotic cells. They contain their own DNA, which is circular, wheras nuclear DNA is linear. They also contain ribosomes for example. If these structures were incorporated into another cell, it would allow the new cell to perform aerobic respiration on its own - greatly increasing its efficiency and giving it a strong selective advantage. It would be a mutually beneficial arrangement - the "host" giving the mitochondria or chloroplasts organic material and the mitochondria or chloroplasts converting it into energy in the form of ATP. Eventually the resulting structure evolved to the point where the individual structures could not function properly without one another

But , like I said, a simple caveate and largely semantic gripe on what is a plant, as zoologically speaking, algae are not always considered true plants.

How did DNA/the nucleus evolve though? For that matter, how did mitochondria and chloroplasts? Creationists like to say that DNA is necessary for proteins and proteins are necessary for DNA, so DNA must have been designed, but what's the real answer? I've heard people say that DNA could have "evolved" from earlier replicators that are independent or semi-dependent on proteins (such as RNA), but how do molecules evolve? Furthermore, I wonder how the nucleus evolved. I know that prokaryotic cells have DNA floating freely within the cytoplasm, while eukaryotes have nuclei. It's easy to see that there is a selective advantage to having a cell with a nucleus, but has anyone documented a change from basic nuclei (perhaps a sexed-up vacuole or something) to the modern, complex one? It doesn't seem probable that a bunch of mutations caused a full nucleus in one jump. Biologists have shown that the DNA in prokaryotes is clustered into a localized area called the nucleoid, which is a plausible explanation for a sort of proto-nucleus, but this still doesn't explain how a nuclear membrane formed. This question is related to the question of how the bacterial ring or lump of DNA evolved into chromosomes of DNA. Lastly, I don't understand how modern cells evolved or acquired mitochondria and chloroplasts. The endosymbiotic theory suggests that those two structures are in fact the remains of ancient prokaryotes which sought a living inside larger cells to process energy and make food for them in exchange for...a place to stay in (I never fully understood what the prokaryotes got in return)? This model explains why mitochondria and chloroplasts have genetic information in them. But what I don't understand is how those primitive prokaryotes "de-evolved" into mitochondria and chloroplasts; prokaryotes are full-fledged (well, almost), living cells, and mitochondria and chloroplasts are organelles. Does anyone know how nature reduced them of what they needed for true life, or are mitochondria and chloroplasts really living cells in their own right?