This brings hope for planet Earth.
Fewer people means fewer pollutants. (Maybe the single biggest factor.) There will be other ramifications (like economic change), but for the planet it's got to be a positive.
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Humans are the first species ever produced on the Earth with the potential ability to save it from the periodic, asteroid induced mass extinction event.
As bad as we are, eventually most species larger than a mouse will get wiped out anyway. The only thing standing between that and inevitability is us.
We can't do that yet. It seems quite plausible that we might be able to do it someday, but if so 5B humans should be just as capable as 15B.
It’s temporary. The humans of the future will be the descendants of the people having lots of kids right now. You should basically never extrapolate out any trends that are driven by people with below replacement fertility rates.
No, it's not temporary. The trend is global and will see the world's fertility go below replacement level soon enough. Of course there are various hotspots, but they'll be smoothed out eventually by emigration or less pleasant means (famine, war, disease).
The three necessary and sufficient ingredients for evolution are: 1) Inheritance, 2) Variation, and 3) Selection.
Let’s consider fertility using this lens:
1) Fertility rate is heritable, 2) Fertility rates vary from person to person, and 3) People with higher fertility have higher reproductive success (it’s almost a tautology).
I’ll let you work out the implications.
That struggles to explain how fertility rates have ever dropped. For higher fertility you would need stronger biologic instincts, the present lack of which suggests they would conflict with something else important for viability, a high "materialist profit" from having having children, which the man's child protection services will catch on to if your group grows big enough, or a "sense of duty", which again seems increasingly hard to maintain as the group grows for an array of reasons.
> That struggles to explain how fertility rates have ever dropped.
No, it doesn’t. The environment we inhabit has changed drastically in the last 150 years. We were well adapted to the environment we lived in 150 years ago. We are poorly adapted to the environment which we live in now. That will change as evolution does its thing.
For a glance into the future, take a look at the habits and lifestyles of people with lots of children. There is an interesting bi-mods distribution, btw.
Well, we are descendants of people who had lots of kids for a very long time. By that logic we should be having lots of kids today, but it's obviously not the case.
The environment we find ourselves in has undergone rapid and drastic change. Many people are poorly adapted to this changed environment. But not everybody.
Their adaptations will spread. And, thus, we shall evolve.