Could a world exist without autotrophs, but have heterotrophs?

Question

I was watching Biblaridian’s biosphere videos and he said it is believed that heterotrophs evolved before autotrophs.

This gave me a cool idea: what if a (my) world had heterotrophs, but no autotrophs? Could the nutrients that original heterotrophs used in the ocean be enough to support an ecosystem on land, or would some sort of autotroph be necessary in order to get enough nutrients for movement?

Edit: someone asked me to clarify my question further, so here I am to answer you questions about my questions! First off- the video is called “Alien Biosperes Part 2” by of course Biblaridian. The time stamp is about 1:08, in which he basically says that “in the history of earth, the earliest organisms were most likely heterotrophic- meaning they were incapable of producing their own energy, and thus had to eat things“ While not said in the video, these heterotrophs (I believe) were throught to get their nutrients from chemicals in the ocean that came out of vents. Ignore the wording of that, as I’m sure it is not 100% spot on. But that is the general idea of it, heterotrophs in the ocean got nutrients from chemicals in vents (and later evolved and broke into autotrophs and heterotrophs.) As for where the heterotrophs would get the nutrients from once moving on to land, well... I’m not exactly sure. Perhaps there are smaller heterotrophs in the ocean that use this method of getting nutrients- that is, eating a sort of “chemical soup”. Then, the food chain could do the rest. “Fish” eat these smaller heterotrophs, something else eats those “fish” and so on. The problem then would be land dwelling animals. Maybe the land dwellers eat sea animals, then other animals eat them. This would mean the whole ecosystem is heavily dependent on sea life. The idea of the whole ecosystem, on land and sea, all being intertwined and interdependent does sound very intriguing though. I fear I have jumped down into a deeper rabbit hole trying to explain this. I hope this helps explain more though!

Answer 1

Not in a normal environment:

A lot of people will refer to organisms like what you're discussing as chemoautotrophs, so it's a blurry line. Heterotrophs need to NOT fix carbon to build organics, which is a challenge. The earliest organisms evolved in a nutrient-rich environment because nothing had ever consumed all the natural organics in the environment. So that chemical soup was fairly short-lived before it was consumed (on an evolutionary scale). Some organisms DO still survive by feeding off the nutrients from thermal vents, but there are a TINY number of thermal vents, and it's likely most of the organisms there were at some point descended from things living elsewhere. There is just not enough sustained energy to lead to the evolution for much more than a few bacterial species. That does not preclude things like tube worms from exploiting niches in these places, but it is hard to justify an entire complex ecosystem based only on these resources. So yes to evolution, but complex evolution is a lot harder to justify.

To make such a system work, you would need virtual "Manna from heaven" raining down on the environment, like a sustained energy harvesting reaction taking the place and resulting in high-energy compounds naturally occurring in the atmosphere. Maybe there are autotrophs elsewhere in the environment shedding energetic particles, but it isn't a good investment for creatures to dump nutrients in the environment. So while you certainly CAN have heterotrophs evolve to significant size given a supply of infinite nutrients, most environments aren't going to provide the sustained levels of nutrients that you would need for millions/billions of years so organisms can evolve to significant size and complexity.

Further, you would need something to prevent the evolution of autotrophs. There are significant advantages to organisms producing their own food and fixing carbon, including [the ability to exploit many unused niches. Even a question like THIS needs a better plausible source of sustained energy than thermal vents.

I'm not a geologist, however, so I suppose there could be specific starting conditions for a planet that cause it to be a giant self-sustaining source of chemical energy (at least until the geothermal power of the planet has all been expended) or a supply of endless organics raining down from some kind of source in space (like some kind of organic nebula). If there are, I don't know exactly what it would look like.