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Suppose a universe with similar physical laws to ours, with the major exception of gravity not existing as we know it, with an analogous force (hereafter called gravity for simplicity) acting as a constant acceleration to the normal vector of an unbounded plane. Given a certain amount of handwavium, a density gradient of material creates layers similar to an Earth-like planet, i.e. molten material, crust, water in some areas, and air. An additional bit of handwavium puts lights in the sky analogous to the sun. Presume an initial condition similar to early Earth, with an atmosphere produced by volcanic outgassing. Then given the appearance of life at an arbitrary point, how quickly would such life propogate outwards from that point, and how quickly would secondary developments, such as the oxygenation of the atmosphere, spread from their respective origins?

EDIT: Include the following additional assumptions. Life begins in primordial oceans similar to on Earth, and that evolution proceeds similarly. Cyclonic weather patterns would not occur due to the absence of rotation, so large scale weather would dissipate more quickly. Tides would result from the same handwavium that puts lights in the sky. Temperature differences from region to region would be more dependant on prevalence of mountains than anything else, but there would still be large scale movements of air and water.

EDIT #2: Tectonic activity is similar to earth, but because of the differing geometry, plate boundaries tend to be mostly straight. Continents would still form but be more angular at their boundaries, and some extreme geography could appear (there's unlimited space for variation after all).

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  • $\begingroup$ Does your life first emerge under water? Is there a weather system? Are there tides and sea-currents? $\endgroup$ – chasly from UK Jan 6 at 17:30
  • $\begingroup$ @chaslyfromUK edited to include additional assumptions. $\endgroup$ – Christyn Jan 6 at 17:36
  • $\begingroup$ Is there tectonic activity? $\endgroup$ – dhinson919 Jan 6 at 17:47
  • $\begingroup$ @dhinson919 yes. Edited. $\endgroup$ – Christyn Jan 6 at 17:50
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    $\begingroup$ If the oxygen produced by the early photosynthesizers can diffuse away forever in every direction, you may never get a sufficient concentration for oxygen breathing life to evolve. On Earth, it took billions of years for oxygen to finally finish reacting with all the exposed iron and other easily oxidized minerals (try reading about the Great Oxygenation Event). In your world it might never happen, since there's an infinite amount of iron to react with (and an infinite amount of atmosphere to fill). $\endgroup$ – Blckknght Jan 7 at 6:10
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Early microbial life on such a world is almost certainly aquatic, so I'll answer this question in those terms.

Earthly bacteria move at a rate between 2 and 200 microns per second, with the higher speeds being attained by organisms with specialist adaptations such as flagella.

The very first cells are unlikely to have such adaptations, so lets assume that under their own power, these cells are limited to the lower value of 2 microns/second.

At this rate, they are able to move 63m/year. Not very far.

Fortunately, these cells can also be transported by ocean currents, which will get them places much more quickly.

A flat world has issues when it comes to ocean currents as there may or may not be differential heating dependent on how the sun analogue works, and there won't be any kind of Coriolis effect.

However, assuming that there were oceanic currents similar to those on Earth, the fastest of these move around 9km/hour, which will get your cells travelling at 78,894km/year.

Keep in mind though, that these currents likely don't move in a straight line.

As a side note, I have my doubts that the tectonic regime on such a world would much resemble that of the Earth. I suspect instead that the mantle would consist of independent convection cells centred over hotspots, and that continental crust would accumulate in the margins between cells. The result of this would be that the surface would be composed of isolated regions of oceanic crust with "walls" of continental crust separating them (looking somewhat akin to a Voronoi Diagram). In this regime, oceanic microorganisms might have difficulty initially moving from one ocean pocket to an adjacent one.

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