I'd like to tell you the story of one of nature's greatest survivors. It's a story that starts billions of years ago when life on Earth was just beginning to take shape. No one really knows what the world was like at that point, but we can be pretty sure that the organisms were very simple. There weren't abundant food sources, and what there was probably didn't provide much energy. There was, however an excess of electromagnetic radiation produced by nuclear fusion just waiting for something to harness it.
I'm talking, of course, about sunlight.
Somehow someway an organism in this primordial land evolved the ability to capture the energy from the sun and store it as carbon compounds. That was a major game changer. It's really hard to overstate how important this was to life on Earth. One could make a pretty strong case that this was the most important adaptation ever in the history of life. Photosynthesis is so common that RuBisCO (Ribulose-1,5-bisphosphatase carboxylase oxygenase, the enzyme responsible for turning CO2 into glucose) is believed to be the most abundant protein on Earth. Give that a moment to sink in... Every cell that we know of has DNA polymerase (to replicate DNA) and RNA polymerase (the first step in making a protein). Photosynthetic organisms make so much RuBisCO that it has both of those beat. As an added bonus this process also produces lots of oxygen in the form of O2. Before photosynthesis there was no free oxygen. Now it makes up 21% of our atmosphere, is essential for many forms of respiration, and reacts with UV light to form the ozone layer.
This newly evolved photosynthesizer (it was probably something like modern cyanobacteria) now had an essentially unlimited source of energy, and it proliferated wildly. So what did the other organisms do in response to the flood of new primary producers? Eat them, of course! Since everything at this point was still single-celled there were no dramatic chases or ambushes, just one cell engulfing another. Somehow someway one of these consumers didn't digest its meal. Instead the swallowed microbe survived, and continued to capture energy from the sun. Now you have the ancestors of the chlorophytes (the green algae). Today this is known as the endosymbiotic theory; the theory that all mitochondria and plastids (the group of organelles that includes chloroplasts) were once free-living organisms.
How do scientists know that?
There are two big clues as to the origin of chloroplasts as organelles. The biggest one is that they have their own DNA, and can divide on their own. Analysis of the genome of the chloroplast reveals that they are very closely related to the cyanobacteria that we see today. The second clue is the presence of a double membrane. The inner membrane came from the original photosynthesizer, and the outer one is left from the original consumer.
Once again, this was a game changer. The consumer already had a rather efficient engine to turn carbon compounds into energy (another endosymbiont called a mitochondria), but now it had a source of glucose and oxygen to use in that engine. These were the ancestors of modern green algae (and land plants by extention). They were so good at making energy that they were able to start forming true multi-cellular organisms with specialized cells and tissues. Oh but it doesn't stop there. You see another group of consumers started capturing and holding on to the photosynthesizers, eventually reducing them to just the chloroplasts, now with three membranes. That's how the red algae came to be. Then it happened again to the red algae, and now there are brown algae with four membranes around their chloroplasts. The moral of the story is that chloroplasts are really handy to have around.
Too bad all of that endosymbiosis happened millions and billions of years ago. We animals missed the photosynthesis train completely. The only way that an animal can get energy from the sun it to eat plants or algae.
Enter the sacoglossa.
Alright if you've been paying attention, then you know that last sentence was not entirely true. Remember these guys? They suck out the cell contents of algae and keep only the chloroplasts. So it appears that animals can still become photosynthetic, we just need to eat a lot of algae (and not digest it).
The real story.
The narratives I've shared here all present the consumer as the "smart" one. Indeed most of the stories that one hears about the evolution of chloroplasts tend to follow that trend. A photosynthetic organism is swallowed by a clever consumer that keeps the prey intact to gain even more food. What if that's not really how it happened? What if the consumers were actually duped?
Think about it this way: a microbe evolves a way to harness the power of the sun. It's population increases so much that it becomes the food of choice for a host of other microbes. Now it evolves a way to survive phagocytosis (being swallowed by another cell). It survives inside its new host and continues to thrive. It's so successful that even when the host is eaten, the photosynthetic microbe survives. The descendents of these early cyanobacteria are found in algae, plants, gastropods and ciliates. Not to mention that free-living cyanobacteria are found all over the world from damp soil to the dead sea.
So the next time you see a plant, seaweed, or green slime on a rock by the water, take a deep breath and think about nature's most successful survivor.