[Iwonda (OP)]

There's a lot of info on how life got started on earth but you seldom hear much about how the food source for that life got it's start on earth. Was plants the first food source on earth or was flesh and is there any studies that indicate which was here first?

[evan_au]

There's a lot of info on how life got started on earth

I would rather say that there have been a lot of theories about how life got started on Earth, but not much real info that would let us choose between them. The following article in Wikipedia lists about 20 theories for parts of the process, not all of them mutually exclusive.
See: https://en.wikipedia.org/wiki/Abiogenesis#Current_models

Was plants the first food source on earth?

Plants are a form of life, so plants were not the first source of food for plants (light is the source of food for plants).

Was plants the first food source on earth or was flesh?

Flesh (meat/animals) is also a form of life, so meat was not the first source of food for animals.

It is thought that the early Earth would have had a mixture of many types of chemicals which would be useful for life.
- It is thought that the first living things would have been consuming this "Primordial soup".
- But none of this soup (or the things which consumed it) still exist, as they would have been eaten by subsequent forms of life like microscopic bacteria.
- And bacteria themselves don't leave much of a geological record
- So to a large extent we are still guessing about the ultimate origin of life on Earth

Some theories even suggest that life may not have originated here, but may have been brought here on meteorites, in which case looking for the origins of life on Earth would be futile.
- In another decade or so, we should have space telescopes which can analyse the atmospheres of nearby exoplanets; this should give us an idea of how common life is on planets like ours.

[alancalverd]

The idea that life arrived from elsewhere doesn't actually address the problem.

If you ask "How does Evan know so much about science?"  "He's from Australia" is not a satisfactory answer. (No irony, honest, guv!)

What the question really means is "how did this complex chemistry arise from simple soup". 

[Bored chemist]

Which came first, life or food source?
Food.
Because food is possible without life, but life is not possible without food.

Obviously this requires the sort of life that can live on simple materials as a food source and that implies that much of the energy comes from elsewhere.
It's possible that the energy source was geothermal, but it's more commonly assumed to be sunlight.
In either case the life is more "plant" than "animal".

[Halc]

Which came first, life or food source?
Food.
Because food is possible without life, but life is not possible without food.

This is the obvious answer.  Any source of energy can be 'food', but life, a consumer of energy, must have an energy source.

It's possible that the energy source was geothermal, but it's more commonly assumed to be sunlight.

Sunlight didn't come in as a food source for a long time, probably at least a billion years after the first life forms.

Most of the pre-photosynthetic life forms that exist today live on methane or geothermal energy sources, but oxygen is poisonous to most of them, so today they only can be found in oxygen deprived areas.  Back in the beginning there was no free oxygen, so the life forms could have evolved anywhere and used other food sources that the ones we see today.

[chiralSPO]

Indeed. As stated above, "food" definitely came first. We could have a philosophical/semantic debate about whether it counted as food before there was anything to eat it, but I don't think that discussion is worthy of this forum.

My understanding is that life on Earth consumed chemical foods like hydrogen sulfide, dihydrogen, carbon dioxide, sulfate, iron salts etc. such as can be found near hydrothermal vents. It wasn't until much later that photosynthesis had evolved.

[Bored chemist]

It's possible that the energy source was geothermal, but it's more commonly assumed to be sunlight.

Yes, I said  "it's more commonly assumed to be sunlight.".
I didn't say the assumption was correct.
The "traditional" view of life getting started on the edges of some primordial sea is still prevalent.

[chiralSPO]

Yes, I caught that "assumed to be" in your post. My intent was to lend credence to the geothermal suggestion.

[Iwonda (OP)]

When thinking about this, I totally left out sunlight and geothermal as a food source for lifeforms. Both of these sources were around long before any lifeforms so with that in mind, I'd say food source came first. It would appear that energy plays a very important role in kick starting life and also in sustaining life.
All of answers given above were very good answers indeed.

[Hayseed]

The "food" was always there.   "food" is a molecule that a life form can absorb for energy and construction/growth/replication components.

The simplest life forms convert raw molecules into bio molecules.  Higher life forms requires "bio food".