All known life forms have life cycles that are not time symmetric as they look different when time is reversed.

I was wondering if there could be a life form that would have a life cycle that would look the same with time reversed? Could such a life form develop intelligence?


Things Which Already Have Time-Symmetric Life Cycles

We call them bacteria. The start as a cell, and die as a cell. That seems fairly symmetrical to me!

Alternatively, you could look at viruses, if you consider them to be alive; they start off as a series of proteins which get assembled according to a plan, go out of a cell, travel around, go into a cell, and get disassembled and turn into a plan, which starts the cycle over again. That seems pretty symmetric to me.

As an honorable mention, there is also the immortal jellyfish, whose life cycle, after a certain stage, becomes cyclical, which can be argued to be time-symmetric. From a polyp they become a medusa, and from a medusa (when certain environmental conditions are met) turns back into a polyp. It's not truly symmetric, as they don't go back into a sperm/egg.

Time-Symmetric Life Cycles May Be A Disadvantage

There is hardly any advantage to going back to a sperm/egg after you've spent so much energy becoming the big, multicellular creature that you are. This is likely why so few creatures do this. It is much more efficient to grow up once, and then pass on genes to offspring as much as you can. Time-asymmetric lives are evolutionarily advantageous; a creature does not waste energy and time going back to a little creature, they just go on until something stops them.

There is an argument that going back a younger state may allow a creature to continue living, because maybe a younger state (like a caterpillar) can acquire food better. This is just not truly time-symmetric, though, because it doesn't go down to a single cell once again. So some back-and-forth within a life cycle could be advantageous, as seen in the immortal jellyfish, but it seems rare on earth.


Now, I like to think there is a nice bundle of intelligence within every bacterium, although they are pretty dumb. Often only marginally better than rocks. Some bacteria can evade predators, signal each other, and some are active hunters, but many of them are living on the whims of chance.

As for more advanced intelligence, it would be unlikely that a creature would retain intelligence as it joins back down into a single cell. It would require some exotic scheme to retain that intelligence, like chemical packets serving the same function as memories or neurons, which seems very unlikely!

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    $\begingroup$ It depends what is meant by "time-symmetric". If the OP is talking about time symmetry and the arrow of time in the physics sense, then your examples wouldn't qualify since bacteria, immortal jellyfish etc. are still taking in lower-entropy "food" and generating higher-entropy waste (including waste heat). And if it has some kind of neural network that can learn, even a fairly simple one like a jellyfish, I don't think it "learns" and "unlearns" symmetrically, the neurons would just die or get re-absorbed. $\endgroup$ – Hypnosifl Mar 13 '16 at 17:58
  • $\begingroup$ @Hypnosifl I briefly considered that, but decided that can't be so because the exact same events for that creature would play in reverse, and the environment for such a creature would't "play along" with that, unless it was a universe where the arrow of time reversed itself frequently. Then there is no point about asking about a symmetric life cycle, because any life cycle would then be "time-symmetric." I would expect someone talking about time-reversing in the physics sense not to say life cycle, but life, and there simply isn't much mystery there. So why would they then ask? $\endgroup$ – PipperChip Mar 13 '16 at 23:23
  • $\begingroup$ Good points, though it's possible the OP just hadn't really thought these distinctions through. If they had, I agree we should assume they were just talking about time-symmetry in some broad macro sense rather than at the level of processes taking place within individual cells "learning" systems like nervous and immune systems. $\endgroup$ – Hypnosifl Mar 13 '16 at 23:26

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