The reason scientists believe this is that there was basically no oxygen in the atmosphere at this time, which meant that cells needed to be anaerobic to survive, going around the problem of no oxygen in the atmosphere.
The first cells had to be anaerobic because most of the oxygen in the atmosphere was produced by living organisms. The first organisms couldn’t use oxygen as an electron acceptor because there wasn’t enough of it to use. Only after later organisms had created oxygen could organisms dependent on oxygen evolve. I don’t know of any scientists who thinks the first cells were heterotrophs. All the ones I know of believe they were chemolithic autotrophs. If you stop and think about it for even a second you will see that the first cells couldn’t have been heterotrophs. By definition heterotrophs rely on eating other living things to survive. How could the very first living thing ever to exist have survived by eating other living things? It makes no sense. The first cell can’t have made a living by eating other cells because there were no other cells to eat. It couldn’t be a heterotroph by definition.
