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I was thinking of a situation in which an Extra Terrestrial plant life form evolves a nervous system and eventually a brain. This plant diversifies into a variety of different life forms with some living in the water, some living on land, and some flying through the sky. It is able to move and its brain controls its movements. It has eyes for seeing, organs that it uses for smelling, and organs for hearing. It derives all of its energy directly through photosynthesis. Most species of this plant have keener senses and a more intelligent brain and the ability to move faster than most animals on this planet.

What type of selective pressure might have caused this plant to evolve a brain and the ability to move?

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    $\begingroup$ Mathematically, unless it had some sort of photosynthesis we haven't ever seen before, the plant probably will not have enough energy to move, especially at high speeds. Even with 100% conversion of light energy to usable energy, I'm not sure it will work. $\endgroup$ – Kyranstar May 7 '16 at 2:49
  • $\begingroup$ Triffids? but that wasn't evolutionary selective pressure. If I remember correctly, it was just an unexpected side-effect of genetic manipulation where the original goal was to create a source of high quality oil. $\endgroup$ – Henry Taylor May 7 '16 at 5:15
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    $\begingroup$ Mostly because it turned into a cow, over 500 million years. $\endgroup$ – Serban Tanasa May 26 '16 at 19:15
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    $\begingroup$ Strongly related question: worldbuilding.stackexchange.com/questions/38394/… $\endgroup$ – Green May 26 '16 at 19:27
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    $\begingroup$ I saw a question related to this a while back. A key question which seems simple but is remarkably hard to answer: what is a plant? As long as you stick to the easy cases of modern day trees and bushes and grass, the line is easy to see. However, when you start exploring more exotic plants, like those developing nervous systems, the line can get remarkably blurry. $\endgroup$ – Cort Ammon May 26 '16 at 19:40

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Let's start with a venus fly trap.

It has a basic nervous system for catching things (which then benefit it with extra vitamins and minerals). Now lets say that the bugs on our planet have a sudden mutation that changes their external appearance and gives them a new body fluid that harms the plants.

The Fly traps will now randomly die...unless they have a way of differentiating. This trait would give those traps an advantage, they survive in higher percentage and thus that system would continue on and refine in time.

Movement might come from the need to follow the bugs. More bugs could mean less need for roots and shorter roots. That could mean a plant that would start with something akin to a lean that would uproot the back roots could end up with moving roots to follow targets.

Add in more advanced leaves to detect bugs and we have proto eyes.

One could assume that it would all expand the plant's basic nervous system, which would become advanced enough to make decisions and we have your plants.

(I just noticed the bit about photosynthesis on a reread) One could also assume that the some of these plants might branch off and use these traits to get more sunlight and fewer bugs. Perhaps a volcano erupted, killed off most of the bugs, increased the quality of the soil and clouded much of the sky. This would kill off the plants dependent on bugs and leave plants that used those same traits to get sunlight while using the rich soil to make up for the lack of prey.

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  • $\begingroup$ Welcome to the site Btuman. $\endgroup$ – James May 26 '16 at 18:34
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    $\begingroup$ As a side note it is probably far more plausible for some of the plants to be immune to the poison than it is for the plant to decide what not to eat. $\endgroup$ – James May 26 '16 at 19:40
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I was thinking of a situation in which an Extra Terrestrial plant life form evolves a nervous system and eventually a brain. This plant diversifies into a variety of different life forms with some living in the water, some living on land, and some flying through the sky.

  • It is able to move and its brain controls its movements.
  • It has eyes for seeing, organs that it uses for smelling, and organs for hearing.
  • It derives all of its energy directly through photosynthesis.
  • Most species of this plant have keener senses and a more intelligent brain and the ability to move faster than most animals on this planet.

What type of selective pressure might have caused this plant to evolve a brain and the ability to move?

So...its a cool idea what you are trying to accomplish.

Unfortunately it is not biologically possible.

Photosynthesis may be the basis of life on Earth, but it is horribly, horribly inefficient.

There is a reason it take acres of plant life to feed a human.

So to address your four main points/design items

  1. Movement is energy intensive and necessitates the development of musculature or a complex system of water pumps to extend and contract various parts of the plant. The main problem here is mobility requires cellular specialization beyond what plants have...and picking up those pieces makes them less "plant" like.

  2. When you start adding sensory organs (sight/smell/etc) you are drastically increasing the amount of energy needed to support the creature. This plant would require a massive amount of space to have the energy, and I would think at a certain size energy transfer would lead to diminishing returns on increasing the size. The size would make item #1 even more impossible as more mass requires more energy to move...

  3. Sorta already covered this. Photosynthesis as we know it is not capable of this. Even man made photosynthesis (solar arrays) is not terribly efficient (though a great technology).

  4. Does not compute. There is simply no way to get a plant that is better at being an animal than an animal.

There is no selection pressure that could get you to this point

Now, that said you could have a sort of hybrid, or rather an animal who retains the ability in their skin cells to photo-synthetically respire. This would be sort of like having an appendix, its a vestige of some past need and is no longer necessary for survival, but it could hypothetically improve a creatures fitness.

The shared ancestors between plants and these hybrid animals would go all the way back to single celled organisms in your reality.

The surface area ratio to mass on a human is way way lower than that of a plant (obviously) so the net energy gain would be minimal...particularly if your people like clothes...

But some smaller animals, perhaps snakes, could get a not insignificant gain from this system.

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  • $\begingroup$ I would think ectothermic animals with higher surface areas, such as from wings or fins or flattened bodies, would benefit more than cylindrical snakes? Mathematically, photosynthesis could support an ectothermic animal up to 30 centimeters in width (Source). $\endgroup$ – Anonymous May 27 '16 at 12:58
  • $\begingroup$ Would it make a difference if the plant was on a planet that orbited a brighter star such as a blue giant and perhaps was in a binary star system? I mean would this still be biologically impossible if the plant was on a planet that orbited a blue giant and perhaps was in a binary star system? $\endgroup$ – Anders Gustafson May 27 '16 at 19:19
  • $\begingroup$ @AndersGustafson Its not so much the amount of sunlight as it is the efficiency of turning sunlight into biologically useful energy (i.e. sugar). Think about this, if you absorb enough heat to make that much more energy could you maintain water in your system or would it all get burned up? Adding more sunlight adds more heat...so...do you end up with a barren planet, or is it a giant jungle...that's starts to get pretty speculative (at least from me and my knowledgebase). $\endgroup$ – James May 27 '16 at 19:26
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    $\begingroup$ @AndersGustafson keep in mind creatures like this exist in all kinds of universes so its not a bad idea it may just not be one that you can legitimately biologically explain $\endgroup$ – James May 27 '16 at 19:27
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There are a lot of good, useful questions already, so I'm just going to add a few ideas that I myself toyed with, working on a sentient plantlike species.

One of the points made a lot in the other answers, is regarding the inefficiency of photosynthesis. Trying to find a way to excuse this also gave me a good excuse to mobilise the plantlife. Two things make photosynthesis so inefficient; the amount of energy collected is not very large, but more importantly, creating chloroplasts is very costly for an organism.

There is a simple way around this. Now, I can't verify how reliable this particular wikipedia article is, but I can definitely imagine this working.

https://en.wikipedia.org/wiki/Elysia_chlorotica

Now this creature is a sea slug, but the basic principle can be used for any organism. Instead of making its own, costly chloroplasts, predation allows an organism to steal the chloroplasts from prey-plants.

When the plants start trying to eat each other, it's easy to imagine them developing movement from that. First the predator will move to better consume the prey. Then the prey will develop movement to escape the predator.

To compensate for the lack of energy gained from photosynthesis, my plantlike aliens will also digest the rest of the prey-plant, giving them the energy required to move like an animal. Perhaps this deviates further from plants than you're looking for, but I figured the chloroplast stealing might still work?

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Plants evolving mobility, eyes and brains is a great idea, but it only works in a fantasy and not in science-based science fiction. Several other answers on photosynthesis show that the energetics won't support plants capable of exceeding animal performance and it doesn't even come close to doing so.

The energy just isn't there. Putting this on a planet closer to its sun won't help either. Plant metabolism runs at only a few percent of animal metabolism. The planet Venus receives twice the solar energy of Earth. Placing the planet closer to its star will only turn it into another Venus. Not the best environment for plants of any kind.

I like the Technically Yes answer where animals have chloroplasts in or on their skins. One possible solution to plants with eyes and brains is that they aren't plants they're really animals camouflaged as plants. Examples are sea anemones in our seas or stick insects.

For evolution to change plants into animals (which is in effect what you're asking to happen and this is most likely impossible or close enough to make no difference) there would have to be an environment which selected for vision, intelligence and mobility. Animals would evolve faster and more readily to become super-animals able to see better, be smarter, and move quicker. Plants wouldn't stand a chance against this competition.

The only plausible way plants could acquire eyes, brains and mobility is genetic engineering. I can imagine a decadent alien genetic engineer spending time on a planet with abundant plant life and to while away the long hours changed the plant life to give it eyes, brains, and a degree of mobility (this might be in only short bursts when the plant stored enough to make a move). However, plant energetics remains a problem. Plant life is low energy life, animals enjoy a high energy biology. Plants able to outrun animals is effectively impossible.

If you want to write a science-fantasy, there are no problems with extraterrestrial plants that are brainy and mobile with eyes to see. Just go for it. The imagery is great and that makes for a great story. Sorry about that, I only wish mobile plants were real they would make life in the universe interesting.

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The "Why" isn't so important. Evolution is fickle; it cares more about the fact that they would survive, than why it would do it in the first place. Look at the Platypus.

A multi-cellular life-form with the ability to use photosynthesis could evolve to be mobile, having specialized cells for movement (possibly levers like muscles, or valves for using hydraulic pressure, like spiders) or some similar mechanisms. It would probably use this adaptation to escape animals that would eat it. As it evolved, it would probably split into several different species. Some might take advantage of actual Plants, and climb up very tall trees to steal sunlight.

Many will probably evolve a lot of surface area to catch a lot of sunlight. However, these creatures would be slow. Especially if they had complex tissues like brains. They would be a lot like some reptiles, not moving much and conserving energy until they need to release a huge burst of speed. On the other hand, there would eventually be races of these mobile-plants, with the ability to eat other creatures. Plants, or Mobile-Plants. Even other animals. They would be a lot different from regular plants though, and definitely different from animals.

If you want plants that can actually "think" and move about freely, then you would want your plants to be able to eat. There's simply not enough surface area otherwise.

On the other hand, you can probably increase their speed by lowering the planet's gravity, or having it ingest materials it could use to produce more energy. Like having them seek out energetic metals like magnesium, to use in aiding their movement.

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  • $\begingroup$ Or perhaps they evolve more efficient methods of photosynthesis, and possibly the local star puts out a lot more useable energy for the process and the natural selection + atmosphere that counteract any added issues to do that extra power. $\endgroup$ – Ryan May 26 '16 at 16:58
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    $\begingroup$ Photosynthesis is insufficient to power animal metabolisms, and moving the planet closer to the sun or increasing efficiency makes negligible difference. An endothermic autotroph could not exceed a small fraction of a millimeter in size, while an ectothermic autotroph could not exceed 30 centimeters in size. Beyond that limit it would have to become heterotrophic, but small flying and swimming creatures are quite feasible (Source). On Earth, Elysia chlorotica is capable of supporting chloroplasts. $\endgroup$ – Anonymous May 26 '16 at 17:55
  • $\begingroup$ @Anonymous I was wondering if an ectothermic autotroph could live in colonies with a queen to lay eggs, and workers to collect sunlight, guards to defend the colony from heterotrophs, scouts to look for the best places to collect sunlight from, members of the colony that raise the grubs, and males that have sex with the with queens to help with reproduction? $\endgroup$ – Anders Gustafson May 28 '16 at 6:51
  • $\begingroup$ @Anders Gustafson: How would the workers transfer energy to the other castes? An autotroph would "eat" dirt and air for raw materials, with energy provided by sunlight. It wouldn't be able to expel on demand the energy it collects. $\endgroup$ – Anonymous May 31 '16 at 12:50
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    $\begingroup$ @Anders Gustafson: Maybe the workers secrete nutrient rich fluid? $\endgroup$ – Anonymous Jun 2 '16 at 16:15
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Human nerve system actually resembles the roots of the tree or a fungus. Something to think about :)

Regarding the need for plants to develop brain and mobility. It could be that without animals to challenge the plants, they would try to compete each other for the sunlight, soil and nutrients. Ability to guard and deny the resources to other plants would mean that plants would develop both offensive (predatory) and defensive (countermeasures) genetics. Once you have predatory plants that try to out compete other plants, they would need to develop strategy, the ones that are not predatory enough would be "weeded out" (pardon the pun). Overtime this would mean that the plants would have to develop a sort of a brain that would be partially based on instinct (action/reaction) and partially on coming up with abstract thinking (strategies, communication with similar species) etc.

The need for the plant to move would come once basic brain functionality is there, because strategically staying in one place would mean resources such as nutrition may be depleted faster and mobility would ensure access to more resources.

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  • $\begingroup$ Generally, the most efficient transport paths from a single point to many points is a tree shape. And I agree with you, it's incredibly interesting to think about. Have a look at the Constructal Law for a more details. $\endgroup$ – Green May 26 '16 at 19:29
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A lot of the existing answers seem to be focusing on the technical aspects of photosynthesis and plant movement, but from what I gather you have either figured that out or don't care (not caring is perfectly valid, a little bit of mystery goes along way) - so I'm going to focus on "What type of selective pressure might have caused this plant to evolve a brain and the ability to move?", which seems to be the actual question anyway.

First, like animals, plants need a few basic things to survive from their environment: water, nutrients, air, and sunlight. The way I see it, the source of these life-elements can vary on a long or short term basis. We see that variation has directly affected the evolution of earths plants. Regionally we see plants that can stand larger variations of temperature and water (areas with more prominent summer/winter or wet/dry seasons), and locally plants that need a lot of sun (like trees) or those that don't (like underbrush).

I'm also going to rule that evolution typically favors survival of the species, not the individual. So a plant being able to roam around must benefit the survival of the species.

Taking all of that into consideration, I'm going suggest an environment where any of the key ingredients that a plant needs to survive vary's too wildly, or for too long of a time for another version of a stationary plant species to survive.

Variation on a slow timescale would promote plant migration to areas where the missing elements can be obtained. I'm thinking Game of Thrones style 9 year winters here, or massive decade long drought cycles. Also, If you want to develop a long history, it could start with ground cover plants that "move" by growing and dying, their root systems perhaps evolving to work more like legs eventually. Familiar mutations related to perception like antenna and eyes would increase competitiveness, and promote more variation, predatory plants, etc.

Variation on a small timescale is a bit more difficult as we see that now with day to day weather, so I'm going to stretch a bit here, but you would need much larger swings of temperature or something that leads to prioritizing which of the life requirements the plant is collecting at different times. A concrete example would be a rocky environment where plants move out onto the rocks to collect sunlight, then back near a warm spring to collect water and nutrients while avoiding being frozen at night.

Basically they would be filling a gap and taking advantage of a source of life-elements that other species can't.

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Technically Yes

If we loosen the definition of 'plant' to a creature that uses photosynthesis and has chlorophyll, then multicellular creature that, before becoming what we consider plants may evolve to avoid things that eats them. It just requires that one mutation of a cell that both uses chlorophyll and moves, and BOOM you will eventually have plant that have brains like animals or plant animals, plantimals. Unfortunately they will not be fast, think a sloth or a turtle on land, but hey both of those survive.

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I have one extremely specific scenario

Underground mazes

There are resource that your proto-plants need to survive , but it is currently in a resource poor aquatic environment. There one thing that your plants depend on to survive. Underground mazes. Basically , as ice forms cracks in rock that is less brittle than average , forming long and deep pockets that sediment may get trapped into, and where water may seep into , storing essential , life sustaining resources the plant needs. The only catch in that the pockets are deep and hard to access, and since your plants' little seeds can waste any energy or resources in this environment, they adapt root like tendrils that first "solve" the maze , then establish the most efficient route from plant to resource , much like slime mold.

After the soil and water are used up , the plant withdraws its tendrils and creeps along the rocks under the same mechanism that it used to use its tendrils to search the maze. After a while , when it cannot find resources , it reverts into a cyst , and floats away , waiting to land on more fertile rock. This process with help a lot when your plants colonize land, but before any of this can happen , the earliest ancestors of these plants must gain mitochondria-like organelles along with chloroplasts through endosymbiosis.

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Recent research shows seeds have decisions-making cells basically analogous to simple brain-cells. They use them to decide when to germinate, in a way somewhat similar to brains.

So, an extremely high concept idea would be to have the seeds evolve into animal-like organisms.

source: https://cosmosmagazine.com/biology/do-plant-seeds-have-brains

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TL;DR: "plant" is a role, and this role functions at timescales incompatible with the original requirement. Anything that can move fast and has a brain is an animal, the best you can get is a photosynthetically capable animal - but the energy it could have from photosynthesis would likely be useless anyway.

The problem that you're facing is that anything that is a plant is incapable of high-energy processes, like movement and heavy, real-time processing (and eventually sentience).

What plants do is concentrate the available energy, and other lifeforms use this concentrated energy to support their high-energy processes. The ambient energy is collected, and with its help, some chemicals that are not of much use by themselves are converted into more energetic compounds (more or less). The energy stored in this way can then power those other lifeforms that move around and respond to things quickly. The waste products are then, of course the same chemicals that the plants "energise".

In other words, plants are not as much as specific kinds of organisms as a specific "stage" of the energy chain - one that concentrates the thinned out ambient energy and passes it onto the next stage.

In Earth's case, the plants mostly work on sunlight and beat stuff like nitrogen, water and carbon into sugars and other organics. Most organisms that eat plants in one way or another can then power their own locomotion (excluding obvious stuff like fungi). Most of those are very delicious and get eaten themselves, but they're still mostly on the same stage, the only really efficient energy boost happens with the plants - the differentiation is really between plants and animals here, as even their cell "templates" are different. Either way, the specifics of the plants we have here on Earth are likely unique to Earth, but the role of that first energy stage is not.

This question brings up an interesting alternative, for example - instead of sunlight, motion energy is captured and (presumably) used and stored in a similar way to our Earth's plants - by converting chemicals into other, more energetic ones. Some of the answers detail very interesting specifics of capturing motion energy, but the overall design and idea of the plant is virtually unchanged, except for specific changes needed due to how such energy is distributed, compared to sunlight (direction, intensity, seasonality).

If those hypothetical plants still produced the same sugars (and weren't poisonous), regular Earth animals could probably feed off of them. We could swap out Earth's plants with those alien plants and it would've probably worked out mostly fine, after like a century or two.

Another thing is that most of the reactions happening in plants are fairly slow, both because of their nature as well as because they happen at the ambient temperatures (that's why warm-blooded animals are more capable than cold-blooded ones), and because of the chemicals and rudimentary signals being sent by free flow of fluids. Animals also have systems that work that way - the different hormonal signals and their effects are far, far too slow compared to nervous system response times (even the fastest hormones in humans are on the order of seconds).

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