If all of us are re-incarnations of prior human beings, how do we account for the growing population? Where do “new” humans come from?
Overpopulation is sure to become a dilemma of human existence in another couple of centuries. Even in the present day it is a major concern to us all. We would all love to escape this forever becoming samsara. But how? Only way out is through the practice and realization of Dhamma. Now, with more population, more desirous things out there, more things to rouse your desire, promote your greed, selfishness, worry, fear, etc. it is actually more difficult to practice the true dhamma. But your question is “where do new humans come from’ according to the scriptures.
When studying the discourses of the Supreme Buddha, there is one thing that is clear to us: it tells us that the beings in the Samsāra have the tendency to ‘reincarnate’ in the four low-level worlds. They tend to get a birth in great hell (Niraya), Animal, Preta (ghost), and Asura worlds. Only rarely would a few go to the higher planes of existence.
If one is to take Buddha’s word on this, only very rarely will a person will get to be born in the human world again. In the Chiggala Sutta: The Hole (SN 56.48) Buddha says:
Monks, suppose that this great earth were totally covered with water, and a man were to toss a yoke with a single hole there. A wind from the east would push it west, a wind from the west would push it east. A wind from the north would push it south, a wind from the south would push it north. And suppose a blind sea-turtle were there. It would come to the surface once every one hundred years. Now what do you think: would that blind sea-turtle, coming to the surface once every one hundred years, stick his neck into the yoke with a single hole?"
"It would be a sheer coincidence, lord, that the blind sea-turtle, coming to the surface once every one hundred years, would stick his neck into the yoke with a single hole."
"It's likewise a sheer coincidence that one obtains the human state. It's likewise a sheer coincidence that a Tathagata, worthy & rightly self-awakened, arises in the world. It's likewise a sheer coincidence that a doctrine & discipline expounded by a Tathagata appears in the world."
We are not necessarily re-incarnations of prior human beings. There are 31 planes of existence in Buddhism. So the present human beings could result from any of the 26 planes out of 31 planes of existence(except 5 non-returner Bhrama realms) which consist of unaccountably large number of beings. For example, imagine the number of beings living in Animals realm (tiracchana yoni).
There is no evidence all of us are re-incarnations of prior human beings. If we were, the Buddha would have explicitly explained this.
MN 38 explains how a new child comes to be:
Bhikkhus, the conception of an embryo in a womb takes place through the union of three things. Here, there is the union of the mother and father...it is the mother’s season and the gandhabba is present....
SN 31.1 gives the impression 'gandhabba' is similar to sperm, i.e., sexual seeds that create life:
Bhikkhus, I will teach you about the devas (gods) of the gandhabba order. Listen to that....
And what, bhikkhus, are the devas of the gandhabba order? There are, bhikkhus, devas dwelling in the fragrant roots, devas dwelling in the fragrant heartwood, devas dwelling in the fragrant softwood, devas dwelling in fragrant leaves, devas dwelling in fragrant flowers, devas dwelling in fragrant fruits, devas dwelling in fragrant sap and devas dwelling in fragrant scents.
These bhikkhus are called the devas of the gandhabba order.
Commentary by Bhikkhu Bodhi:
The gandhabbas are associated with fragrant substances, no doubt because the word is based on the stem gandha, meaning scent.
In the Pali language, the word for 'human' is 'manussa', which means 'high minded' or 'noble/good heart'. The word 'manussa' is not the same as 'puggala' ('people').
SN 56.102-131 explain 'animals', 'ghosts' & 'hell' beings are reborn 'human' & 'godly' by seeing the Four Noble Truths and also explain gods & humans pass away into hell, the animal realm & the animal kingdom by not seeing (adiṭṭhā) the Four Noble Truths.
This question is born out of the misconception that Buddhism teaches reincarnation as in after death you can only become a human (I've seen this same question and criticism of reincarnation on many websites before), this portrayal in the media is completely wrong and inaccurate.
The Buddha says:
"Sariputta, there are these five destinations. What are the five? Hell, the animal realm, the realm of ghosts, human beings and devas" (Maha-sihanada Sutta, MN 12)
Those are the five possibilities for anyone's prior existence.
Modern day humans were not necessarily humans on Earth in their previous existence (actually quite unlikely), they could've been in a hellish world, an animal, a spirit, or in a heavenly world in their very last previous existence.
This means a growing human population does not contradict rebirth or re-existence (the world population was 3 billion in 1960, now it's more than 7 billion).
The Buddha for instance was not thought to have been a human on Earth in his last existence, but existing in the Tusita heaven before taking birth on Earth. He had a great childhood, good-natured parents, and lots of luxuries which few humans even in modern times have.
In short all "new" humans in their prior existence before taking birth as a human existed in a hellish world, as an animal, a ghost, a human, or in a heavenly world (and very unlikely to have been a human right before this existence).
It is just a popular misconception that Buddhism teaches that after death one is only born as a human on Earth again and again.
UPDATE: The Buddha seems to explain that the majority of humans and devas (heavenly beings) do not take birth as humans after passing away:
Then the Blessed One, picking up a little bit of dust with the tip of his fingernail, said to the monks, "What do you think, monks? Which is greater: the little bit of dust I have picked up with the tip of my fingernail, or the great earth?"
"The great earth is far greater, lord. The little bit of dust the Blessed One has picked up with the tip of his fingernail is next to nothing. It doesn't even count. It's no comparison. It's not even a fraction, this little bit of dust the Blessed One has picked up with the tip of his fingernail, when compared with the great earth.
"In the same way, monks, few are the beings who, on passing away from the human realm, are reborn among human beings. Far more are the beings who, on passing away from the human realm, are reborn in hell... in the animal womb... in the domain of the hungry ghosts."
"In the same way, monks, few are the beings who, on passing away from the deva realm, are reborn among human beings. Far more are the beings who, on passing away from the deva realm, are reborn in hell... in the animal womb... in the domain of the hungry ghosts." (Pansu Suttas, SN 56.102-113)
The stereotype portrayed in the media of Buddhist reincarnation (after death one is born as a human again) is here stated as extraordinarily rare by The Buddha himself.
The majority of humans and devas after death go to hell, the animal womb, or the realm of ghosts instead of being reborn as a human or a deva in a heavenly world.
So based on what The Buddha says I would expect that the majority of "new" humans came from hellish worlds, the animal realm, or the realm of ghosts since The Buddha explains how extremely unlikely birth as a human is after death as a human or deva.
But the number of existences is countless so everyone existed in one the five realms many times before which may cause confusion.
The Buddha also explains in the Cula-kammavibhanga Sutta (MN 135) how one can exhaust their good and bad kamma in a human state instead of in a heavenly or hellish world.
The Buddha also explains in the Maha Kammavibhanga Sutta (MN 136) how it's not true that all good people go to heaven or that all evil people go to hell and that some evil people go to heaven and some good people go to hell by explaining the different variables involved (one's past kamma, later kamma, views at the time of death).
I still don't know where the popular media portrayal view of rebirth only as a human comes from debunked by The Buddha himself.
Birth as human is really rare, usually more pleasant than many other destinations, and gives one many opportunities.
I'm not an expert on this subject by any means, but based off of what buddhist texts/commentary I have read, I'll try to answer the question.
When you ask the question, you assume that whatever the "quantity" for human life is, that it is finite (e.g. 1 "unit-of-life per person) ( I know that's kind of a reduculous comparison, but I couldn't think of anything better). However, life isn't finite and so, theoretically, so long as there are enough physical recourses, there never would come a time when life simply can't occur due to too much already existing life.
My next point is that there are two "entities" of our lives. The first is the one that we know, have experienced, and remember. It is our life according to out memories and basically what it is that makes us different on the surface from someone else. On the other hand, every living thing also posseses its "life force". It is from that that we derive karmic chains, karma from "past lives" and really just karma in general, as well as the ten worlds. This too is from where we derive, "the nature of our lives".
What is important to know is that our, "life force" is existant as a single entity in everything. Not only that, but, its existance is constant. Our life force is eternal in that it has existed since the infinite past and will forever exist into the future. (Sorry if right here I'm getting a bit confusing. I'm having a hard time explaining this, so please feel free to ask clarifying questions) What I'm trying to say is that, were the life force a concious being just as we are, it wouldn't precieve time nor space. The past is occuring right now, in its present, just as the future is. All of time and space exists within the "life force" that I'm trying to explain. Just as a photon, since it is going at the speed of light, "precieves" no time, the same is true of the "life force". To it, something that happened a countless number of years ago is, and forever will be, happening at the same time as you are reading this.
It is for this reason that in one of the first paragraphs I put "past lives" in quotations. We don't really have past lives in terms of past recolections. We do though in terms of our life force which has existed since the infinite past. This is what is meant by the fifth supernatural power that the Buddha has which is, "the ability to see past lives". Basically, what the Buddha really was able to do is precieve the nature of his life, and therefore "see" past lives (this is where the karmic chain stuff I said earlier becomes relevant). So we are, as living beings, more physical manifestations of our life force than we are our own singular entity, which would mean that there is no physical limitation of our life. And that therefore, the population could reach quadrillions and we still will have been "reincarnated".