Question

Were the first animals carnivorous, herbivorous, or something else?

Answer 1

Since plants had to appear first, to allow animals to have anything to eat, then the first animals were likely herbivores. This is simplifying things a lot, but until plants got here, with their ability to make food from CO2 and water, there was not much happening on the Earth, maybe just a few bacteria like life forms feeding on things like Iron or Sulfur (the oxidation of these elements give off energy, and some bacteria in some really harsh environments, like deep sea vents utilize these sorts of energy forms to make ATP, but that is beyond the question.).

 

After plants, or at least photosynthesizing organisms got here (think blue-green bacteria,, what we used to call blue-green algae), some life forms began feeding on them. Or at least, this is likely what happened.

 

Once animals began feeding on plants, a food chain would have developed, where some animals began to feed on other animals that ate plants, in a situation of a sort of energy transfer from simple compounds (CO2 and water -> glucose -> amino acids/protein etc.) to more complex ones, and energy from primary consumers (herbivores) to secondary consumers (predators) to tertiary consumers (folks who eat folks who eat meat).

So, the first critters on Earth were likely herbivores, and were followed by carnivorous folk sometime later.