Pre-industrial humans were as carbon efficient as today's natural gas (methane) burning power plants but with a couple of important differences

The basal metabolic need of an adult not engaged in any energy intensive activity is ~ 7 MJ/day. This amounts to ~1700 Cal (kcal)/day which explains why our the food labels are based on a daily diet of 2000 Cal/day. This is also ~ 2 kilo Watt hours  (kWhr) per day.

We exhale about 1 kg CO2/day

This means the CO2 intensity of our minimum daily subsistence is about 500 gCO2/kWhr

Of course, if we just subsisted by consuming food that is hunted or farmed organically with animal inputs (manure and ox for ploughing etc.) and water from rains and rivers diverted by water wheels and gravity then we are not adding any net CO2 to the atmosphere or oceans which was how it was in the pre-fossil age.

The enthalpy of combustion of natural gas or methane (CH4) is 890 kJ/mole, which translates to 55.62 kJ/gram or MJ/kg of methane. 

CH4+ 202->CO2+2H20

The combustion of methane emits 2.75 kgCO2/kg methane which translates to ~50 gCO2/MJ

However, the efficiency with which the heat energy released is converted to electricity in a modern gas turbines (without waste heat recovery or use for Combined heat and power) is about 37%, which means effective emission intensity of natural gas plants is = 481 gCO2/kWhr  (Calculation: 50gCO2/MJ *3.6 (MJ/kWhr)/0.37)

Therefore, pre-industrial humans were as carbon efficient as natural gas power plants with one important exception, the carbon they emitted was biogenic, i.e., was sequestered during photosynthesis and will be again while natural gas combustion adds to the atmospheric carbon stock. 

Another difference is that a typical gas power plant can be about 500 MW which means they generate 10,000 MWhrs/day (operating full capacity for 20 hours a day). This is the equivalent of BMR of 5,000,000 humans (Calculation: 10,000,000 (kWhrs/day)/ 2 KWhrs/day/human) 

While we could do stuff like capturing the carbon emitted and storing it underground or extracting it from the atmosphere called direct air capture but these come with a huge energy burden. As to the energy requirements for these, I will discuss in a future post. For now let me just say I am reminded of what the economist John Maynard Keynes is to have said in the context of what the government should do when stuck in a depression "If necessary, in the name of stoking demand and people’s expectations, “The government should pay people to dig holes in the ground and then fill them up.” Continuing to burn coal and then using energy to suck carbon from the air might end up amounting to that or worse for it would not even employ a fraction of the people manual hole digging and filling would.