In the 1800's Malthus hypothesised that at some point we will run out of resources. With the introduction of GMO's, this problem no longer seems to be an issue for humanity. But what if some major catastrophe were to completely destroy the production or growth of genetically modified crops, animals, foods, and seeds. Would we be able to feed eight billion people with purely organic (natural) food? Would it be possible for humans to be as powerful and abundant as we are without GMO's? What actions would humans take to try and sustain 8 billion people without GMO's?
[I'm going to upgrade the question because really it has some questions worth pondering by everyone here.]
That's three separate questions really, one about Malthus's assumptions, the next about our sustainability and the last about what happens were anything happen to GM crops. GM food has gone mainstream and begun creeping into the consumer chain slowly, as I realised when researching this answer. Soy lecithin, vegetable oils based off canola and cotton, corn, corn syrup and corn starch and several other basic components of processed foods have a high probability of being GM already. (The next time you buy a pack of biscuits or chocolate, check the contents please.) While anti-GMers focus on frakenfood as a possible source of health issues, this question touches on what would happen if we started depending on the few food species that would dominate agriculture due to the ease of growing GM foods and then something happened to them.
Malthus first: Malthus lived in a time where population seemed to be growing exponentially and food supplies growing linearly if at all. His theories seem outmoded and pejorative today for the very right reason that his basic assumptions no longer hold. It is not necessary to convert more land to agricultural use to produce more food; merely increasing the efficiency of existing land can get more results. Population doesn't always grow exponentially even if all factors including food, healthcare and socio-economic stability are highly conducive and it does not require catastrophes or overt measures to reduce it to 'sustainable' levels. Indeed, the problem that most developed countries world over face today is the demographic shift towards an ageing population without adequate young people to replace the numbers.
In a way, we humans have been modifying plant and animal genes for thousands of years through selective breeding for efficiency. GM, at its basic, tries to speed up this process, replacing trial and error methods by genetic engineering. Even without resorting to GM, agricultural scientists have been able to produce high-yield, short-cycle foodcrops and livestock. The green revolution was fundamental to sustaining the exponential human population growth after the 1900's though it came with its problems of increased crop variant/strain monoculture, overuse of pesticides and fertilizers and the attendant environmental degradation.
GMs partly came as response to those problems, where you didn't have to do a constant escalating battle between increasingly resistant pests and bigger amounts of pesticides. GM does increase the tendency of monoculture that already set in from the time of green revolution to the point where we are dependent on just a few crop species to feed all our people. Loss of crop diversity is bad - it represents the loss of genetic base material for that specie as a whole and increases its vulnerability to factors such as new diseases or super-pests. This is why seed vaults, some amount of traditional methods/crops and alternative food movements are important even if they seem inefficient - they preserve and sustain crop diversity.
So finally to the question: Without GM, it is still possible to feed all our people. As the comments also noted, we also have a massive problem of food wastage and inefficiency. About a third of all food produced goes to waste due to its looks, inefficient transport and storage and even due to periods of local oversupply. You could feed around 2 billion people on just this!
We do have urban farming, hydroponics, vertical farming, glasshouses and other modern food production techniques that are similarly designed to increase production efficiency without escalating inputs. The only issue with these is the heavy setup costs.
I suppose you could move into the realm of science fiction in the future and see glasshouse environments producing food in giant space stations and planetary colonies. There is no end to human ingenuity and will to persevere really.
So yes, it is possible to sustain our population food-wise without GMO though easier with GMO. The bigger problem is when we use up our non-renewable resources such as oil and coal.
GMO's are fairly recent development, if you look over the last 50 - 60 years you'll see that world hunger / food insecurity has declined significantly. The developed world actually has the opposite problem - there is a wide spread obesity epidemic.
If you look at a map of food insecurity you'll notice it's prevalent in some of the least developed parts of the world (parts of Africa, SE Asia, central America. With modern growing techniques (fertilization, proper irrigation, etc) the farmers there can double and triple their yields and eliminate food insecurity.
The main problem with food nowadays is not production, but distribution; and, in your given scenario, distribution would still be a problem, but it would change:
- A) Where food production is difficult/impossible, there would be starvation.
- B) Where food production is possible, but unexplored, production would likely be encouraged and increase (i.e. in gardens).
- C) Where food production is possible and explored with GMOs, food from the same sources would be more expensive, considering humanity would never drop profit-driven production.
What's really interesting here is what happens in C) - such radical change would affect the economy severely, and I believe a culture of self-production would spread. The current profit-driven production system would fight back, trying to maintain the concentration of resources. Once people realize that they can survive without the supermarket, there would be a war.