Is it possible for there to be a planet where no animals, only plants existed? What evolutionary reasons would cause this, if possible?
The Earth had life without multicellular animals for roughly three billion years, so it should be pretty obvious that such a planet is possible. There were certainly plants for some of that very long period — the amount of time depends on your exact definition of a plant. But there has been photosynthesis for nearly as long as there has been life.
Mike Scott observed that the necessary conditions already prevailed on earth, so I will throw out some ideas on why they might have stayed that way.
Anything which hinders locomotion would select against animals. A high gravity planet could make the cost of locomotion higher than the benefit. There is some question about plant propagation and dispersion, but the gravity should only slow this, rather than prevent it outright.
It's generally presumed that you need oceans to get life in the first place. Perhaps you planet has oceans, but they are small and few. Animals generally started in the water, where buoyancy helps with locomotion. If you make the oceans shallow, plant life could simply make locomotion in the water infeasible or costly. Think giant kelp forests covering your whole ocean.
A world which had been seeded with only plants could meet your criteria. Imagine a world like the ancient earth, but empty of life. Terraformers seed its oceans and land with bacteria, wind-pollinated plants and photosynthetic organisms. Perhaps they add some fungi to help break down dead material.
Animals might eventually evolve from those starting materials but it would take a billion years. If you visited twenty million years after the seeding, you would find a quiet, green world.
Other answers have done a great job explaining that this could indeed be very possible. I would like to point out a couple of things to consider about a world that has evolved with only plant life:
No food plants
"Edible" plants like fruits, vegetables, nuts, and nutritional root plants have evolved mainly to spread the seeds of the parent plants through being carried (Internally or externally) by animals. While still technically possible with the lack of animal life, this would be useless and possibly even a negative trait so anything edible simply will be unlikely to have evolved, or in a very different manner than earth.
No toxic/poisonous plants
As this is mainly used as a deterrent for animals our outside sources, toxins and poisons are fairly useless as an evolutionary trait. Instead, the plants would select more positively for aggressive traits against other plants, like fungi or ivy.
If you were hoping to build a beautiful world full of fruits and veggies, beautiful colours and sights, ready for your protagonists to peruse, again, no dice I'm afraid. As you're aware of the birds and the bees, having flowers to attract bees to pollinate is fairly useless without bees. Most plants and trees would look very boring indeed.
Due to the lack of animal life, self-pollination would be the way to go. Therefore plants would exist in very localised pockets of similar plants, as there won't be much means for the plants to travel and spread. The only common plants you'd see would be aggressive weeds. It would be unlikely for example to see the same plants on either side of a wide river.
Yes - in the common-language sense of the terms, there can be a planet with only plants.
Firstly, ultimately, the energy to run the life cycles comes from such things as a nearby star or thermal vents (that is, the thermonuclear reactor in the planet). Plants on earth both use carbon dioxide and light to produce oxygen, and use oxygen in the manner of the typical animal - producing carbon dioxide. The energy source is not the oxygen, but, ultimately, the sunlight. This can be seen as a cyclic reaction (just like the Belousov–Zhabotinsky reaction), that in this case is kept going by a complex of enzymes and membranes in the bodies of the plants. This does not require that the required reactions occur half in one type of organism and half in another. The whole cycle can be completed within one solar-powered organism.
In common language - plants have stiff tissues and don't move around. Animals have flexible tissues and do move around. But, there is nothing against the idea of a photosynthesising animal - its just that a solar powered creature has less motive to move around. And like a penguin no longer flies, such an animal might spend all its time sunbaking. Consider cold blooded creatures that do exactly that. They might not photosynthesis, but they are certainly solar powered in a rather direct sense.
I suspect that whether moving creatures would always evolve is a more difficult one to answer. But, one argument is that moving creatures evolved because of periodic resource problems that could be solved by flopping into the next puddle. If that circumstance never eventuated, then perhaps no particularly large and mobile creatures ever evolved. Another reason for moving creature evolution is to graze. If you specialise in consuming the creatures next to you, then likley eventually you will find yourself on your own - at which point it is a good idea to move a bit. And then you get creatures that specialise on eating creatures that move, and the arms race is on.
But, if there is abundant solar light, planetary heat, and volcanic chemicals - perhaps motion, in and of itself was never that useful, never immediately the best option to evolve in that direction.
In response to a comment. The above discussion does not commit itself whether a cow-like animal could power itself from the sun, nor does the point being made require this to be resolved.
The core point is simply that the entire chemcial cycle could exist in one organism - that obtains its power from the sun and absorbs only non organic chemicals from the air, water, and dirt. I would expect that such a creature would be extremely sedentary - qualifying in common language as a plant. And not requiring any mobile animals to complete the chemical cycle.
The evolutionary side point is that there seems to be an opening for an organism to move around at least a little to graze on the nearby plants. And this could lead to a creature that gains more and more of its energy and chemicals from other creatures. If this evolutionary trend was available, one would surely end with mobile creatures killing each other. (You see, evolution can only end in tears - back to the trees with the lot of you!)
Of course, if you posit that the genes for mobility were, for some, pehaps contingent, reason unavailable. Then these plants would be no more likely to evolve legs and walk than a human is likely to evolve jet propulsion and fly. I would suggest that it is unlikely but plausible that a planet could have got caught in such an evolutionary niche that might at least last for millions of years.
But, I must make a comment on solar-powered cows. A mobile creature the size of a cow could power itself with solar power alone, as long as it did not move very much. Imagine a creature that sits in the sun for several days before getting up and moving to the other end of the paddock.
It is not the size of the cow that is the issue - it is its highly mobile internally heated lifestyle. And it pays for that lifestyle in having to eat constantly. A cold blooded sedentry animal, for example, some kind of tortoise, might be as large as a cow, but eat and move much less.
If you mean Earth-plants, then no, for many reasons:
- Plants are eukaryotes - that is, a symbiosis of archaea and bacteria, neither of which are plants (or animals).
- There is evidence to suggest that land-plants have depended on symbiosis with fungi from the very beginning; fungi too are neither plants nor animals.
Thus, we certainly couldn't have plants without archaea, bacteria and fungi. We don't know enough to tell if animals are essential for the survival of all plants - there are certainly many plants that have evolved co-dependency with animals (think 'flowers and bees') - it appears that seed-producing plants and insects have evolved together, so if all animals were to disappear, it would probably mean the extinction of any plants that produce seeds.
With the reservations above in mind, if by 'only plant life' you mean 'no animal life', then I think one might imagine this, if something could have selectively killed all animals while sparing most plants at some very early stage, certainly before seed-bearing plants evolved.
What that would be is a difficult question - my best guess is that there would be something in the physical environment that favoured plant cells and made animal cells unfeasible. Perhaps this: Plants almost routinely produce viable offspring with multiple copies of their genomes (look up diploid, triploid etc), whereas animals can't; perhaps this gives an evolutionary advantage in a high-radiation environment, where there is a high risk of having your genome shot to pieces.
On another planet evolution would probably have taken a different course, although the principles would be the same: the initial players might not have been like bacteria and archaea, there might have more, life might not have been organised into cells and so on. On the other hand, some of the major steps in evolution on Earth would be universal - symbiosis, for example, whether it is in the form of bateria living inside an archaeon (as in eukaryotes), or plants forming mychorriza with fungi, seems to always give a strong advantage. Both the phenotypes of 'plant' and 'animal' must have been hugely advantageous for them to become so dominant on Earth, so something similar is highly likely to evolve elsewhere - and if only the plants evolved, there must have been something in the environment that ruled animals out.
Sorry for rambling on like this - I hope you could use some of it as inspiration :-)
You'll need to whittle down what you mean by "Plants" vs "animals". Like AlexP states do you mean only autotrophs (animals that produce their own food)? Do you mean only animals derived from autorophs (ie an alien venus fly trap) do you mean only organisms that appear plantlike (fungi are not autotrophs). You address that other "animal" life must be simple, so at least the bacterial distinction vs plant is not an issue, so bacterial symbiotic relationships still work even with earth plants. To have this happen you would basically need to make it so that large multi-cellular heterotrophs don't give significant advantage to single celled heterotrophs. Making it so that plants cannot be consumed by heterotrophs as a source for food is one option, there by stunting the available food resources for all heterotrophs.
I believe it is possible for large multi-cellular heterotrophic life to simply not exist. But what you will eventually be left with afterwards is suspect. Plants use CO2 and O2 to live, using the carbon from CO2 as their primary structural material for everything. Producing lots of O2 can result in oxygen poisoning in other life possibly destroying symbiotic relationships with bacteria, or even killing themselves. Likely plants would fight through this, and fires would be far more likely (releasing CO2). Plants would then have another issues.
- Surviving fires
- Getting more CO2
If plants keep encountering fires, plants will either need to incorporate fires into their life cycle (as some modern trees do), or be built to simply survive fires (as some modern trees do). Some conifers actually encourage fires with dry needles in order to kill other plant competition. With this you might have a situation where plants don't even die with the fires, keeping CO2 stuck inside the plants instead of the air, stunting growth of all plants. Plants that have quick life cycles or life cycles that incorporate their own destruction by fire to spread seeds would have the advantage of quick adaptation to the ever changing landscape caused by the atmospheric imbalance of primarily having autotrophs, but plants built to survive from the get go might out-compete these plants by simply not providing enough room for them to exist.
If CO2 is removed from the atmosphere, then plants can't grow. So depending on the decomposition rate and CO2 released that had been sequestered by plant growth, plants may need other strategies in order to get enough carbon to actually live. One way to accomplish this is parasitic activity. Some plants already act as parasites to other plants, normally trees, and derive some of their nutrients from them. Conceivably then, you could even have plants that eat other plants entirely in order to get CO2 or other nutrients, since simply extracting it from the air is not a good long term solution.
Presumably, in such an large animal free planet, you would have a bunch of "plants" that could withstand fires, have quick life cycles to quickly adapt to the changing environmental situation, and plants that essentially kill and take over other plants. This would then cause an arms race for "predator plants" vs prey plants, and this may result in what we would call "animals" all over again, at least on the predator side, and maybe only at the "seed" stage which allows them to move to new locations, or simply let seeds be light/tiny enough to go through the wind. The parasitic/predetory behavior of these other plants would likely make them look more like fungi or sessile cnidarians with similar life styles, as feeding on other structures may give enough energy that autotrophic activity is not necessary.
This planet would probably look like a bunch of tough tree like structures with a web of multi colored mycelium and vines, and "potato eye" parasitic structures covering everything, probably with a haze of a pollen like substance in the air and lots of fires starting . Some of your plants will probably become parasitic or predatory at some point, and stand a high chance of evolving into what you would might call an "animal" at least at some stage in their life.
I can't believe no one has mentioned this but if you have low levels of energy on the planet, that almost precludes everything but plant life. Animals, especially motile, multicellular animals consume a lot of energy and if the food you eat takes more energy to obtain than what you get from it, you just starve to death. Only sessile animal life might have a chance such as that during the pre-Cambrian. There are also other forms of multi-cellular sessile life that are neither animal nor plant such as lichen and fungi. Makes for a fairly uninteresting planet though.
The thing about plants and animals is that you need all kinds of complex relationships for an ecosystem to be stable. Without animals, plants will have a harder time spreading pollen and seeds, for example. Animals also keep plant populations in fine control and provide manure.
The way to go for a plant-only planet is to have some plants taking the role of fungi and animals. Perhaps in your world, animals evolved with the capacity to take root and photosynthesize. The reason they behave like animals, eating plants and other animals is because photosynthesis is not enough to cover their energy expenses - if cows could photosynthesize, for example, it would cover only 4% of their caloric needs.
Such "plantimals" could be so related to plants, for example, that they would have cell walls and other cellular features characteristic to plants.
A couple examples of such creatures in literature:
- The florans from the Starbound videogame are a race of intelligent spacefaring plants that come from a plants-only planet.
- Palmon comes from a digital universe, but she could be a good template for a "plantimal".
We also have many questions about plant sentience here at World Building. You could have a look at them for further inspiration.
Something non-evolutionary (possibly).
See the Alien: Covenant storyline. A bioweapon (black goo) was deployed to eradicate a civilization. Only black-goo plantlife survived, puzzeling the visitors in the movie.
He leads the crew to a temple in a city full of dead humanoids. David tells them that upon his and fellow Prometheus survivor Elizabeth Shaw's arrival at the planet, their ship accidentally released a virus which annihilated all fauna on the planet, and that Shaw perished when the ship crashed.
Plants and Animals are only two of the 5 Kingdoms. But let's eliminate bacteria, virus's, and everything else we can't actually see. We are left with things we can see, which are animals, plants, and fungi.
So your hypothetical world without animals could exist, I think, with fungi transporting plant spores through the air. Already many plants rely on the wind and fire, etc to fertilize themselves. With a developed and developing fungi, developed because there has been no animals to take their place, they could even be trying to take the place of plants in many cases. And through evolution, a possible fungi analogue to animals may happen.
This isn't just possible, it's similar to what our own planet's land masses would've looked like during the Devonian Era. It's weird to think about it but animals evolved significantly earlier than plants which were not differentiated until they colonized land and it took a while for us to follow. Animal/Plant interactions lagged behind other inter-Kingdom interactions, sure we see a lot of co-evolution now but it wasn't really necessary. All you need is enough fungi and microbes to make enough CO2 and some decomposers (which is generally the same organisms), both of which bacteria do to a large extent do and probably would've evolved to keep pace with trees if animals hadn't. Large trees would take longer to decompose without insect mandibles or even be buried first. This would also potentially lead to lots of coal deposits is that matters to you since those are the circumstances under which our coal was mostly deposited.
I'd recommend spending some time reading up on this Natural History to get a sense for it:
The Late Devonian Extinction; one of the theories explaining this event was the plant-mass produced a sudden uptick in O2.
The Carboniferous Era; The era when we saw the biggest effects of lots of plants and not enough decomposers, also concentrate on these plants