evolution question

Here is a question for you evolution experts. (I don’t want to turn this into a creation-evolution debate, so take that elsewhere.)

What is the theory behind why some animals evolved into carnivors while others are herbivores? Wouldn’t it make sense that all animals would evolove into herbivores? Here’s my thinking… any animal can find plants… there is no hunting involved (other than in places where plantlife can’t exist). It can’t run away, therefore it would seem that it would be what animals would gravitate to. Now, once all the plantlife is gone, I can see where a fast animal would track down a slow animal and eat it, but I would think that it would still be able to eat plants. Also, what would cause an animal, like a lion, to be wired to be a carnivore? Does internal chemistry of an animal force it to search for meat as opposed to plants for nutrition?

I don’t know why I think about these things. I just do.

A few general comments:

  1. All animals are heterotrophs – they get their food energy from outside their bodies, rather than producing it themselves as plants do. (Yes, some animals like coral have close symbiotic relationships with plants, but the coral cells themselves are still heterotrophic.)

  2. Carnivory evolved in marine environments in (I believe) the Ordovician. Not a lot of plant life down there to eat. Even today, the vast majority of sea creatures eat other sea creatures.

  3. Darwin postulated that if a niche exists, a life-from will evolve to exploit it. Living tissue (plant, animal, or other) is a good source of nutrients. Thus, all animals are walking food lockers. It only makes sense that some creature would evolve to take advantage of this resource. Indeed, it would be strange if they didn’t.

Eating land plants is difficult. With so much of their mass being in the form of cellulose and other indigestable fibre an organism needs to process a lot of foodstuff to get enough to eat. Other animals are much more compact and easier to digest. When vertebrates first were forced onto the land the only thing they could eat were the herbivorous arthropods that had proceeded them. Millipedes and suchlike. The adaptation needed to digest plant material did not come about until much later.
Consider the difference in features of a lion’s gut and an antelope’s. The lion’s is far more primitive, showing many fewer adaptations and derived features. Meat eating is the basic design for mammals.

Dr. F:
Good answer…Very thought provoking.

Cellulose is not “indigestible” per se, it just that we don’t have the enzyme required to break the particular glucose linkage that’s found in cellulose. Certain bacteria do have this enzyme; I’m not sure if any higher animals do or not (I think termites are symbiotic with bacteria to enable them to digest wood).

DrF, are you implying that animals preceeded plants on land? If so, why couldn’t plants succeed on land without animals? If not, why couldn’t the animals eat the existing plants? (if there was plenty of food, why didn’t the herbivores go get it?)

Here’s a possible reason: Carnivores need much more physical agility to survive (they have to kill other animals). That agility and aggression might have helped them make landfall, so to speak. Essentially, they carried their sea battles onto land.

I suppose that means the very first land animals were herbivores running from carnivores, but getting eaten anyway. Thus carnivores get the credit as the first successful land animals.

Is all of this completely up the wrong tree?

I thought Dr. F meant that the first animals on land were herbivorous invertebrates. They had the necessary enzymes and/or friendly bacteria necessary to digest cellulose. They probably evolved this ability while paddling around in shallow ponds and marshes eating vegetation, and crawled out on the land to avoid competition from other spineless vegetarians.

Thus they paved the way for the first land vertebrates who might not have adapted to plant eating. I imagine the first vertebrates to crawl out on land were somewhere between lungfish and frogs - amphibians that pursued their arthropod prey out onto the shore.

This is all pretty vague in my memory so forgive any factual errors I have made.

As to the question of whether or not plants could survive on land without animals - I think that plants could survive without animals, but they wouldn’t be much like one we know them today.

There would be no sexual flowers, which rely on insect to pollinate them. The insects, of course, rely on flowers to feed them. They’re good friends.

There would be no alkaloids. This means no mescaline, no THC, no nicotine, no morphine, no cocaine, and gasp no caffiene. All these drugs are neurotoxins to insects, and have, … er … other effects on mammals. Plants evolved them as defense against herbivores - and the most relevant (numerous) herbivores are insects.

There still might be trees, though, since growing tall was a way for plants to compete with other plants for sunlight. Then some shade-loving plants evolved to take advantage of the dark forest floor.

Boris pretty much has it. The earliest tetrapods (vertebrates with four legs) were basically shallow-water fish that lived in weedy swamps. They were derived from the lobe-finned fish (ray-finned fish had not yet developed). the fringy parts of their lobe fins were not as effective in their environment as fringe-less lobes would be, and if they could develop fleshy nubs on the ends of their lobes to help them push through the weeds, so much the better. They would push themselves through the weed-choked shallows looking for worms and crawfish equivalents. Shallow water like that has very bad oxygen levels in hot weather, so those that lived there also perfected the ability to extract atmospheric oxygen through a pouch off their gut. When these swamps dried up, some of these fish found they could survive (for a while at least) out of the water, their knobby lobe-fins could push them around on land as legs, and the air-pocket was a lung.

Now, plants had figured out how to live out of water quite a while before. And the grazing invertebrates followed the plants. The land snails, millipedes, worms and the scorpions and spiders that preyed on them were not so very different than the snails, worms, and crustaceans the tetrapod-fish were eating already. The land belonged to plants and arthropods before those ugly fish crawled out to eat the “bugs”, just as it still does.

Dr. Fidelius, Charlatan
Associate Curator Anomalous Paleontology, Miskatonic University
Homo vult decipi; decipiatur

After all, what has more of the nutrients an animal needs than… another animal?

They’re like giant vitamins.

Torq said:

Hence the evolution of fast food chains? :wink:

Very interesting stuff. Thank you everyone for your input. Dr. F., you say that “meat eating is the basic design of mammals.” Is this true? Not all mammals eat meat, do they?

Also, I can buy the idea of meat-eating evolving from sea creatures and their subsequent move to land. But here’s the question… with the abundance of plant life on earth, it would seem that animals would have evolved into herbivores (obviously, some of them did). Conversely, if other animals are such good sources of nutrients, it would make sense to me that they would all evolve into strictly meat eaters. Quick land animals like the antelope are swift enough to chase down other animals, so why did they evolve into herbivores at all? Are you saying that the first animals out of the sea were herbivores, and that the ability to eat and digest plants is much more “evolved” than meat-eating, therefore those animals that evolved into herbivores were “stuck” and couldn’t re-evolve into meat eaters?

I guess I’m curious as to why there seems to be a need for the mix. Dinosaurs also seemed to have this mix of carnivore and herbivore (although I don’t know if any of them ate both… kind of hard to imagine a T-Rex grazing). When they were wiped out, evolution seemed to force this same arrangement in nature to occur again. Interesting…

I think it’s just a question of specialization. Herbivores often have specialized grinding teeth and stomachs. Most carnivores have sharp teeth that would get quickly worn out on a vegetarian diet.

Vegetal matter is very abundent but is much harder to digest. Also, it does not contain as much energy as meat. Herbivores spend much more time eating than carnivores. It makes sense to specialize as a meat eater, though more and more meat-eater have been found to eat some plant matter from time to time.

Only humans commit inhuman acts.


There is only one generality that you can make about why animals do what they do. That generality is that there is no general rule.

What “seems to make sense” would only hold true if the ecosystem were designed. Any given animal is only interested in living long enough to have offspring. Whether the method that animal has “chosen” to do this is good for the ecosystem as a whole or “makes sense” in a broader context is irrelevant. Because animals which cause the collapse of their local ecosystems tend to run out of places to live, nauter looks to be in balance. All the unbalanced types have died off already. (Of course, if a new way of unbalancing everything comes along, like toolusing communicator apes, all bets are off until a new equilibirium is found.)

To restate, animals are not interested in what makes sense. Their only concern is to live long enough to ensure that their progeny survive. The various methods of achieving this tend to balance each other out, and questions about why all animals do not follow the same path are interesting only to philosophers.

Dr. Fidelius, Charlatan
Associate Curator Anomalous Paleontology, Miskatonic University
Homo vult decipi; decipiatur

Let’s not forget one important factor, although I don’t how it fits into the evolution theory – if all animals were herbivores, population control would be a BIG problem. Well known fact – where there are no natural predators, you get a runaway population. We have that problem in the D.C. area with deer. The Park Police in Montgomery Co. actually had to kill a bunch of 'em because they were eating up so much vegetation (caused a lot of controversy too, as you can imagine).

If I understand things correctly, the growth of any population (including the deer in DC)will only go as far as the ecosystem can handle it. My guess is the population explosion of deer is because the vegetation can handle them. If not, they would be starving to death to reach a natural equilibrium. Large deer populations are usually weeded out because of the problems they cause the human population around them.

Perhaps the fact that herbivores and carnivores co-exist IS a natural result of evolution and population control. But the idea that the world couldn’t have ONLY herbivores surviving quite nicely (the population in balance with the food supply) still escapes me a bit.

But I’ll get over it.

Ecosystems quickly fall out of whack without control. As the above two posters mentioned. Around O’Hare in Chgo, we have a lot of problems with Canada Geese. They have no ememies and breed out of control. I have even seen them downtown.

Preditors can help keep herds healthy. For instance if one deer is sick he can infect the whole herd. BUT if the wolf eats him, one life is lost but the herd is spared.

A good way to think of evolution is where there is water, there will be life, where there is life, there is other life to eat that life. And so on…(yes I know this overgeneralizes a bit but you all get the idea)


So then we’re we’re all set up for a vegetarian-carnivore debate, right? Oh.


I think it was the Flower Children of SF that restated that for public consumption as:

“If it feels good, do it.” It lasted longer than I thought it would; but, although apparently viruses are being discriminated against through exclusion from the lifeform circus, HIV managed to cramp the hippy niche eventually.


Cannibalism is then the ultimate meal? But I understand the New Guinea highlanders got into trouble with that – when they picked up a brain disease.

/o. . . /
_______/\ . . . ?
. # . #

So why ya stickin’ aroun’ a’ready? Ain’tcha spawned?

Of course, if a new way of unbalancing everything comes along, like toolusing
communicator apes, all bets are off until a new equilibirium is found.{/quote]

Yup, that’s us – communicator apes being used by their tools. How cell-phonish to our fellow DNA-game players.

Except, of course, those who feel they need to restate things.

To err is human; to restate is charlataneous.

And BTW, do Charlatans come from Charlotte, NC, Charlottesville, VA, Charlotte, VA, or Charlottetown, PI?

(Charry 'bout that.)

Boris B:

So I need only my caffeine to rid myself of bugs, and don’t have to trip out on all those other things, I guess? :wink:

Sofa King:

The same animal. . .but we already ingested and digested that here.


. . .if all animals were herbivores, population control would be a BIG problem.

OK, so, since it isn’t, we’ll just keep on. . .(I think the term is something like ‘truckin’.)

But hey, now, none o’ youse considered us carnivorous plants, e.g., Venus’s-flytraps? We woulda ruled this here planet if online supermarkets had come along a little sooner. We thought we had the perfect niche in the market. . .until all those megaCORPorate mergers took place.

DrF, I’ll save you a few calories: Here’s a free one: “[sigh]”


Ray (omnivore – enough meat here for the month)

But for one tiny speck toward mankind (shoulda been):
/o. . . /
. # . #


Let’s get this FINnished up:

/o. . . . /
. # . .#

Ray (slow, but still somewhat ahead of evolution)