I suspect this is a typo as 200 million years ago
the world is habitable. I believe that it was the start of the dinosaur era.  Perhaps what was meant was 200 billion years ago.

I think the scenario as presented with bacteria
entirely populating the sea represents the earth
before much life was on the planet.

Toxic gas from anaerobic bacteria, toxic, causes mass extinctions, turns the oceans pink.

...Scientific American had an article about this about a year ago, the idea, I believe, is that as the oceans warm up, cyanobacteria (thus the purple?) come up to the surface and spritz sulfur dioxide, which rains down on everything and destroys most life...or did, about 90% of all species, although most orders (mammals, reptiles, dinosaurs) made it through.

My first child will be born in less than three months. "Saving the Genus" really doesn't do it for me.

What we are facing is a tipping point that may well set life back several hundred million years. At the least we are facing a civilization-disrupting unbalancing of the earth's climate, meaning the unpleasant deaths of billions from warfare, starvation and disasters such as floods, hurricanes, etc. in the short term (present lifetimes).

The task for the portion of humanity that is aware of these threats should be to spread two messages: one, that this threat is real and present, and two, that this threat is solvable and presents an opportunity to change our society for the better.

While George Mobus is correct that saving the human species may not be possible, his comment makes a basic assumption that needs to be challenged:  Should the human species or genus be saved?  From the point of view of the rest of the planet, the answer is a resounding NO!!!

The evolution of a species that relies for its survival on an overpowering intellect, coupled with the physical ability to make major changes in the physical characteristics of the planet with their opposable thumbs, has been an absolute ecological and environmental disaster and catastrophe.  The results have been destruction of every major ecosystem on Earth, unnatural pollution of every inch of land, air, and water, a mass extinction caused by one species, constant unnatural noise and light, and such gross overpopulation by one species that other species have no room to live properly.

Homo sapiens had a lot of potential but, unfortunately, took a wrong turn long ago, resulting in the current mess that's likely unfixable.  Human intellect has run amok, totally unguided by wisdom or integrity that are both needed in order to properly control the human mind.  A good analogy would be a car with the throttle all the way down but no driving who can either let off the throttle and put on the brakes, or even steer.

The "self-conscious, abstract symbol processing sentience" that Professor Mobus wishes to continue is properly evolved as beings who are barely noticeable, not as a species that resembles a cancerous tumor on the Earth.  A highly evolved species would barely be noticed, and would certainly not be overpopulated, consuming much, or causing the ecological or environmental problems that humans are causing.  A good example of this was a Star Trek episode called, "Errand of Mercy" from the original series.  But alas, with a few rare exceptions like Buddhist monks, this is a Utopian ideal that will never be reality.  Unfortunately, it's obvious that the extremely powerful intellect that's needed for Professor Mobus's self-conscious, abstract symbol processing sentience must dominate the other parts of the mind that could control it.  The sentience that Professor Mobus wishes to continue thus comes at far too great a cost to the rest of the Earth.

So, the best thing, by far, for the rest of the planet is that Homo sapiens become extinct, and the sooner the better.  Environmentalists who really care about the planet should be working to greatly reduce or eliminate the harms caused by humans, not to ensure survival of the human race.

The task for the portion of humanity that is aware of these threats should be to spread two messages: one, that this threat is real and present, and two, that this threat is solvable and presents an opportunity to change our society for the better.

Roboc1

As for #1, I spend much of my waking time doing just that. As for #2 I spend the rest of my waking time searching for the answer to a single question: Is the predicament that we have created solvable?

The truth, a hard truth for many people used to seemingly miraculous technological solutions, is that there are problems for which there are no feasible solutions. This can be mathematically proven. The other truth is that no one knows that our very real problems are solvable or not. We have lots of opinions and beliefs. And there are some reasonable sounding approaches (giant concentrating thermal solar collectors in the deserts). But there is not a person on this planet that can say (with evidence to back them up) that these problems are solvable, let alone what the solution(s) will be. We face not just one threat but many, all brought about by our excess numbers and excessive wasteful consumption. We are not (yet) a wise species.

That doesn't then translate into 'therefore we shouldn't try'. Quite the contrary. We should try very hard, hence your message #1 is right on the money. But we should also recognize that we may need a backup plan in the event of a worst case scenario. That would be wise.

George

George Mobus, Associate Professor, Institute of Technology, University of Washington Tacoma, and Professional Student for Life