Tornapart, it’s a good question. I’m a Christian still, and except evolution as true now but didn’t when I was a witness, so I understand, I think, where your question comes from.
From an evolutionary point of view, species normally reach equilibrium with their environment. It’s a like an arms race between the species and other organisms around and the environment. Normally this equilibrium is stable but can get out of balance for the good or bad of the species. Most of the time it is quite stable though. The longer the time frame ones looks at the ups and downs of the stability the more obvious it will look. When some organisms for example mutates, it may have an advantage as with bacteria or viruses. They might be able to inhabit humans where they could not before when our immune system was adapted to them, and many in a population can die. A few will normally survive because all humans are a little bit different from one another and the population will increase again, but this time with the inherited difference a few survivors had spread to all their descendants and so the process repeats. This is but one of many things that impact on human reproduction. The result is that human populations were quite stable in the past. This was also the case before humans became human, and after, even though there was no magic line to cross that meant human from natures view point.
It is important to note that with any arms race, one side as it were, will never greatly outsmart the enemy so to speak. This means that the likelihood of population increase to any great degree is limited by all the surrounding forces that kill off people and offspring. Occasionally an environmental or biological development will occur that kicks stability off into the negative side of things, and extinction or near extintion can happen. The superior forces won as it were. However the positive side of imbalance can happen as it did will human beings when we started to develop medicine and farming. In very recent times both faming and medicine have been greatly boosted by science, the roots of which are about 2 or 3 thousand years old. This is why humans have now become so numerous.
If all this is correct it makes a prediction or two. That prediction is that nature should be trying to close the gap between our advantages we have through science to survive, because the forces of nature are the other side of an arms race and natures every changing lifeforms and environments. This prediction is born out and we see it in many ways, not least in the world of disease caused by bacteria and viruses. Aids is a very real and relatively new threat, along with the fact that antibiotics are not working as they once did. Bacteria is becoming immune to them. So the arms race continues but before science, humans had no advantage to positively increase over and above equilibrium. Another prediction of all this is that a few humans should be immune to big threatening diseases that come along, like aids for example. The bigger the population the more likely this is to be the case, and this has been observed in a few individuals who survive aids and are not infected by it long term. If it were not for science, aids and many other biological type threats would have wiped out all humans except for those special individuals who were naturally immune, and from them a new generation with that immunity would have sprung, and this is how it worked until relativity recently. Intelligence and science are such powerful advantages if they are continually used, that the human advantage may now never completely be equalised by nature’s arms race. I hope my little article was helpful to you.