Picture, if you will, four planets sharing the exact same orbit and orbital speed, and separated by exactly same distance from each other. Each planet is the same size and roughly the same gravitational pull and are stable in this arrangement. No satellites and each has no axial tilt.

If each world's surface and atmospheric features were similar, what could cause their evolutionary paths also to become very similar?

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    $\begingroup$ even species separated by a cliff can have different evolutionary paths $\endgroup$ Commented Oct 13, 2014 at 11:45
  • $\begingroup$ I'm fairly certain there are many processes involved that are very chaotic, so I see no reason why teh evolutionary paths on practically identical planets should be even remotely similar. $\endgroup$
    – overactor
    Commented Oct 13, 2014 at 11:45
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    $\begingroup$ Not directly what you're asking; but without some form of active intervention to stabilize them the arrangement you're describing won't be stable over mega-year timescales. Even if there're no other planets in the system to perturb the 4 planets orbits, other stars passing by will generate a tiny disruption in the spacing/timing of them. From there, the oscillations will feed back positively until you're playing interplanetary billiards. $\endgroup$ Commented Oct 13, 2014 at 13:08
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    $\begingroup$ I was going to point out the instability as well. Especially if there are any other bodies in system. If the central star weren't perfectly spherically-symmetric, that might also create slight imperfections in the gravitational field that begins to induce perturbations. And as soon as that symmetry is broken, things will continue going down hill. I do wonder, though, whether it might actually be slightly more stable if you had six planets, equally spaced, each placed at the Lagrange points of its neighbors. $\endgroup$ Commented Oct 13, 2014 at 14:38
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    $\begingroup$ @CalebHines that would be a good question for Physics ;-) $\endgroup$
    – David Z
    Commented Oct 13, 2014 at 15:57

3 Answers 3


There are two concepts at play here. Speciation and Convergent evolution.

Speciation is the evolutionary process by which new biological species arise. I won't go deep into the details as the question asks how they can be similar but suffice to say that Speciation is the explanation as to why evolution can create difference branches even in identical situations, given this, and the fact that mutation is generally randomized, there is a statistically negligible chance the the same mutation would occur upon all the planets.

Which brings us to my answer. The only way in which to ensure that evolution would be similar between the planets would be for someone to interfere with the genetic structure of the creatures on the planets. The only reason why its not impossible is because you asked for similar evolutionary paths. Can you imagine how difficult it would be to DNA sample every single insect, plant and creature on four planets to ensure they were all identical and then cull those that were not?

An alternative non-evolutionary interference would be that the creatures do not reproduce but are instead grown and released into the ecosystem at a steady rate.

But what about convergent evolution? This theory talks about the independent evolution of features, that is to say several different creatures developing the same kinds of abilities. Given our understanding of evolution (and constrained by my lesser understanding) it would be fair to say that you would still encounter inserts that appears to the casual observer to be doing similar things. Smaller lifeforms are so abundant and different that you would be hard pressed not to have something similar to ants, something similar to pollinators, etc. Plants would still be green (Assuming earth like condition, see this question.

I feel that generally you would still find the basic concepts, as survival of the fittest tends to mean that the most efficient way always wins out, so they would be common across all planets. Where you would start to encounter differences would be in the more complex life forms where you would notice changes. You would still find predators and prey, you would still find creatures that fly and creatures that swim in the seas. At this point, it all depends on how similar similar is as to if the answer is yes or no.

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    $\begingroup$ That is enough reading up on evolution for today. I might go back and read the evolution articles on Wikipedia further at some point. Made me learn something today. $\endgroup$
    – Mourdos
    Commented Oct 13, 2014 at 12:29

Since you are asking for similarity...there are only three options:

Convergent evolution

This answer here gives an excellent explanation of one way you could end up with similar life forms: https://worldbuilding.stackexchange.com/a/2496/49

You can expect that animals similar to great cats, grazing beasts, fish, monkeys and birds to evolve on any planet. They may have differences but they would have more similarities than differences. They most certainly would not be the same species or able to crossbreed though.

Cross planet transfer

Some agency is allowing animals and plants to travel from one of the planets to the other. Whether this is weird alien technology or unusual astronomical phenomena a substantial amount of life must be passing from one planet to another.

Consider just how divergent life just on two islands on earth can be and consider the divergences between different planets. Even if they all started off the same they would not remain the same!


Some force is deliberately shaping the process. This is actually made more likely by the fact that the arrangement of four planets as described is highly unlikely to have formed by accident.

  • $\begingroup$ Upvoted because Star Wars actually uses Cross planet transfer at one point: the sister planets Onderon and Dxun share atmospheres briefly during the summer, allowing flying mammals to migrate between the two planets. This would help with a convergent evolution process. $\endgroup$
    – Mark
    Commented Oct 20, 2015 at 12:48

Adding a different explanations to the good existing answers:

Common origin of life

As you have not defined how life came to exist on these four planets, I offer the idea of a common origin: maybe at some point during the genesis of this star system two big bodies collided, at least one of which contained the first proto-life form which was the basis for all life on your planets.

This event could either be the fundamental origin of the 4 planets - however it is unlikely, that any living thing would survive such an event. Alternatively it could just mean that the four planets where pelted by small meteorites which impregnated them with the first proto-life.

From there on you would have evolutionary processes as described in the other answers, but at least based upon some common proto-ancestor. This could provide some basic guidelines, which might give the four different ecospheres a bit more similarity than completely separate origins of life.


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