I've sat through (heck, even instigated!) plenty of lengthy discussion about various definitions of life. But just now, I'd like to propose a brand new definition: life is anything that pollutes the environment. What do you think?
At first, it always seems like a good idea. Photosynthesis, for example. Here we are, just some simple cells swimming in water, bathed in sunlight, with plenty of carbon (mostly thanks to the volcanos*). Let's take those raw ingredients and make sugar! Incidentally, we'll produce a little oxygen on the side. Shouldn't be a big problem. Right? Wrong! It turns out that a little oxygen becomes a lot of oxygen, and oxygen is hugely reactive. Right off the bat it started oxidizing iron.
Once, iron was dissolved abundantly in the oceans, and the carefree phytoplankton probably never gave a second thought to the fact that they needed (a miniscule amount of) iron to build their photosynthetic machinery. But these days, almost all of the iron in the world has been transformed into iron oxide, rendering it virtually insoluble and therefore inaccessible to the very phytoplankton that need it. They must instead rely on oceanic fertilization by rivers and blowing sand from the Sahara. Bits of the ocean that don't have much riverine input and aren't downwind of any deserts are notoriously iron-poor. The low chlorophyll (indicative of low phytoplankton activity) in these areas was a big puzzle to oceanographers until they figured out the iron connection!
Jumping from single-celled goo to the mammals, let's consider gray whales. Our penchant for categorization compels us to divide whales in two broad ecological categories: toothed whales (think killer & sperm**) that eat seals and squid and maybe the occasional whaler, and baleen whales (think blue and humpback) that swim gently through the sea like giant Brita filters, absorbing plankton. But there are more than two ways to feed yourself from the ocean, and gray whales have gotten creative.
Gray whales are baleen whales, but rather than swim placidly through the water, filtering as they go, they've decided to dig for their meals. They're bottom feeders, foraging through the silty seafloor for crustaceans and worms. Now, if you've ever gone diving, you know what happens if you kick your fins too close to a sandy, or worse yet, a silty bottom. Clouds of dust billow into the water, obscuring your vision and taking a frustratingly long time to settle down. Now, imagine a whale deliberately rooting around in the muck! Gray whales re-suspend huge amounts of sediment, filling the ocean with dirty plumes that are easily visible from aerial surveys.
Sloppy eaters are always attended by those willing to pick up the crumbs, and gray whale mud plumes provide superb "ephemeral foraging opportunities" for seabirds (fun citation!). Millions of birds feed regularly off of gray whale plumes, making these environmental disturbances easily comparable to a human landfill or elementary school (at least from a gull's perspective).
My point is that all living things, from whales to bacteria, alter the world around them to some extent. Put a plant in a jar and it will eventually use up all the carbon and starve. Put a rat in a separate jar, and it will breathe up all the oxygen and asphyxiate. But put them in the same jar, and they can coexist for a little while--long enough for the rat to starve to death, or maybe eat the plant and then asphyxiate. Hmm.
Okay, add more plants, some insects, and eventually you'll find a balance of consumption of production, death and spawning. You'll have created an ecosystem.
So that's the game of Life: it changes its environment, pollutes it, if you will, and then some other Life jumps on the pollution as a resource. Meanwhile, we humans are the Protean player, the ecological wild card. We're something special in the sheer variety of transformations we can accomplish. Physical, check (we build dams). Chemical, check (greenhouse gases and holes in the ozone!). Biological, check (I can't even decide which species to pick--dodos? moas?--but we can eat pretty much anything out of existence). In addition to making a wide variety of changes, we also make changes very fast. We're changing things faster than we can even learn about the way they were before we started changing them.
Which is really pretty spooky.
* Yeah, so obviously this definition of life renders the Earth itself a living being. Volcanos belch ash and gas into the atmosphere and alter the global climate. Does this put me on the same page as Gaia theorists? And then there's fire . . .
** Sperm whales are weird! And not just in name. They are most definitely endowed with teeth, not baleen, but a guy named Milinkovitch proposed in 1993 that they are actually more closely allied with the baleen whales. He did some genetics to back it up, other people did genetics to contradict him, and people are still arguing about it. This should be interesting even if you don't really give a fig about genetics, because it also lets people argue about echolocation, which is undeniably cool. Toothed whales (including sperm) echolocate. Baleen whales don't. If sperm whales are truly more closely related to baleen whales, then either (1) sperm & other toothed whales evolved echolocation independently, or else (2) the first whales evolved echolocation, and baleen whales lost it somewhere on their evolutionary trajectory. Everyone agrees that (1) is extremely unlikely. It seems there's some evidence that embryonic baleen whales may actually have a residual "melon" (à la bottlenose dolphins) early in development--which would support (2).