I decided that I wanted the world to be warm, wet, and mostly land, but didn't know what I was doing and I paid the price later. After the initial formation of the planet I massively ramped up volcanic activity and sent massive numbers of meteors crashing into the surface. Tectonic activity was very high, and the atmosphere was a hellish, ridiculously dense soup of N2, CO2, and CH4 with surface temperatures well exceeding 50o C even around the poles. When the planet finally cooled enough to form proper oceans I found that the surface was about 70/30 on land and sea. The tough conditions meant that prokaryotic life took longer than usual to form, about 1.5 billion years (compared to Earth's 700 million,) with eukaryotes not appearing for another billion years. Complex multicellular life, in the form of simple anemone analogues, appeared just 250 million years later. These and other radially symmetrical life forms spread extremely rapidly through the oceans, as bacteria gave way to vast jungles (which given the world's age I can only assume were made up of giant lichen and horsetail things) on land. Around 100 million years later those jungles were swarming with simple arthropods, and algae-eating mollusks were starting to displace the jellyfish in the planet's southern ocean. just 50 million years later, I had my first mass extinction. The jungles had by then converted most of the CO2 in the atmosphere to O2, and the atmosphere was a disastrous 35% oxygen. There weren't enough terrestrial organisms to establish a carbon cycle and the oceans were too small to serve as an effective sink, so the ecosystem collapsed as uncontrolled fires completely destroyed the jungles and almost everything that lived in them. Only a handful of islands near the equator were spared. The planet's main landmasses ended up dominated by grasslands whose CO2 needs were much lower, and weren't recolonized by animal life (mostly in the form of enormous insects) until a large island that still retained the old ecosystem became a subcontinent. A few terrestrial, tri-radially symmetrical organisms appeared as well but the insects quickly displaced them from the mainland and again the only surviving populations were on some isolated islands away from the Pangaea.
200 million years later the insects were no longer the dominant life form, having been replaced by reptiles, some of them quite large, and some salamander things in the coastal regions. The oceans were now filled with fish, although mastery of them still belonged to the mollusks and arthropods and would from that point forward. By now I was expecting (and hoping tbh) to get sapient dinosaurs but hilariously it was the primitive tri-fold isolate population that finally made the leap, with stone age tripod societies quickly spreading throughout their Madagascar-sized island which was now in the middle of an inland sea. They stayed there for a long time, almost 400,000 years, but they eventually reached a bronze-age equivalent tech and were able to make the voyage to Pangaea. They quickly established colonies along the coast and a golden age ensued. There was war and famine though, and a lot of them regressed to band societies after a while. The center island was again the source of progress as an industrial revolution kicked off there. These peoples conquered all those who didn't industrialize themselves, but it took a very long time. Long enough that the pollution of an industrial society became a problem. It wasn't like Earth, there were no permanent polar ice caps in this world and the O2 concentration in the air was so high and CO2 so low that the climate was actually stabilizing, at least in the short term. The problem was airborne particulates. Try as I might I could neither get them to advance to a level of technology that would prevent the catastrophe, nor (as I later tried to do) cause them to regress to a pre-industrial state and allow the atmosphere to recover. They actually put enough particulate matter into the air to cause an effect analogous to a volcanic winter. The planet cooled significantly, and desert claimed most of the planet's landmass. None of the tripod people survived the desertification or the wars and unrest that broke out as society's energies had to be directed toward immediate survival and social institutions could no longer be maintained. There were still some reptiles around though, and besides the extinction of the radiate organisms (due to glaciation of their habitat) the ocean was more or less unaffected. I could've kept playing, but the tripods were my doods and seeing them wiped out kinda bummed me out.
SimEarth.