You probably don't appreciate plants enough. It's OK — none of us do. Given that plants have been the major player in the convoluted soap opera of life that landed us on this planet, we should be thanking our leafy friends every day for our existence.
Honestly, the whole story is so tangled and complicated, we may never know the truth about how our green ancestors allowed everyone else to evolve, but one aspect of the story certainly involves photosynthesis — a plant's ability to make its own food out of sunlight.
Photosynthesis: The Key to Life
"A great way to appreciate photosynthesis is to compare Earth's atmosphere with that of our 'sister' planets," says Gregory Schmidt, professor emeritus in the Department of Plant Biology at the University of Georgia. "All three planets were most likely similar when they formed and cooled, but the atmospheres of both Venus and Mars have 95 percent carbon dioxide (CO2), 2.7 percent nitrogen (N2) and 0.13 percent oxygen (O2). Earth's air is 77 percent N2, 21 percent O2 and 0.41 percent CO2 — although that number is rising. That means there are 800 gigatons of carbon dioxide in our atmosphere, but there is another 10,000 gigatons — 10,000,000,000 tons — missing or buried in the form of fossil limestone, coal and oil."
In other words, carbon has been smuggled out of the atmosphere and into Earth's crust for billions of years, which is the only reason this planet is at all habitable by multicelled organisms.
"So, how did that dramatic atmospheric shift happen for Earth?" asks Schmidt. "There's only one answer, and it's pretty simple: Photosynthesis, the most amazing factor in Earth's evolution."
A Green Revolution
PHOTOSYNTHESIS, friends. Around a billion years after the Earth was formed, life showed up — probably first as some anaerobic bacteria, slurping up the sulfur and hydrogen that came out of hydrothermal vents. Now we've got giraffes. But there were 10,000 gigatons of steps on the road between the first bacteria and giraffes: Those ancient bacteria had to figure out a means of finding new hydrothermal vents, which led to the development of a thermal-sensing pigment called bacteriochlorophyll, which some bacteria still use to detect the infrared signal generated by heat. These bacteria were the progenitors of descendants that could make chlorophyll, a pigment that was able to capture shorter, more energetic light wavelengths from the sun and use them as a source of power.
So, in essence, these bacteria created a means to capture the energy of sunlight. The next evolutionary leap necessitated working out a means of stable energy storage — creating a sort of sunlight battery that encouraged protons to accumulate on one side of their internal membranes versus the other.
Burning Water (Photosystem II)
The true wonder of plant and algae evolution is in the fact that, at some point, these ancient chlorophyll-producing bacteria started generating oxygen. After all, billions of years ago, there was actually very little oxygen in the atmosphere, and it was toxic to a lot of early bacteria (it's still toxic to anaerobic bacteria that remain in the oxygen-free places on Earth). However, the new process of capturing and storing sunlight required the participating bacteria to burn water. Yeah, they burned that stuff that firefighters use to put out fires.
The process of burning is just oxidation — the ripping off of electrons from one atom and the transfer of those electrons to another (which is called reduction). Early photosynthetic bacteria developed a way to capture photons — basically particles of light — and use their energy to strip water of many of its protons and electrons to use for energy production.
The Breakthrough of Breakthroughs happened 3 billion years ago was when photosynthetic machinery was perfected to the point that chlorophyll could split two water molecules at the same time — these days we call this a "Photosystem II chlorophyll-protein cluster."
Green Batteries (Photosystem I)
Cyanobacteria evolved once these photosynthetic bacteria figured out how to burn water and store the energy from that chemical reaction. In photosynthesis, Photosystem II (water burning) can't really be sustained without the second stage, Photosystem I, which involves taking the electrons swiped off the water molecules in the first step and making use of them before they decay. Photosystem I does this by sticking these electrons on a chemical assembly line so the organism is able to retain that hard-earned energy, which is then used to convert CO2 into sugar for the bacteria to use as food.
The Dawn of Chloroplasts
Once Photosystems I and II were sorted out, cyanobacteria took over the oceans, and because oxygen was their waste product, it became plentiful in Earth's atmosphere. As a result, many bacteria became aerobic — that is, they required (or at least tolerated) oxygen for their metabolic processes. About a billion years later, protozoa evolved as anaerobes (an organism that doesn't need oxygen for growth) scarfing up aerobic bacterial prey. At least once, the bacteria was not completely digested, but stayed within the cell and ended up helping the oxygen-intolerant anaerobic organism cope with the aerobic environment. These two organisms stuck together, and eventually the prey organism evolved into a cell organelle called mitochondria.
A similar scenario occurred with cyanobacteria around 1 billion years ago. In this case, an aerobic protozoan probably gobbled up a cyanobacterium, which ended up setting up shop inside its host, resulting in a small, membrane-bound organelle common to all plants: the chloroplasts.
As algae and multicellular plants evolved and benefited from plentiful CO2 and increasing oxygen in Earth's atmosphere, chloroplasts became the place where photosynthesis — Photosystem I, II and even more complicated stuff — went down in each cell. Just like mitochondria, they have their own DNA and spend their time busily harvesting light for the plant, creating the entire foundation for life on Earth.