> wouldn’t it have emerged by now through natural selection?

It could very well be that more efficient photosynthesis is actually detrimental.

Making everything faster/stronger/lighter at all cost is a human thing, nature is about balancing thousands of variables, not optimising the shit out of a single aspect while ignoring the rest.

There already exist plants adapted for optimal yields at lower CO2 levels. They are classified as C4 plants and hit optimal photosynthesis at about modern levels. These include corn and sugarcane, but only make up a small fraction of overall plant species (about 5%).

C3 plants hit optimal levels at higher concentrations, usually 1100-1300 ppm (90% of plants fit this category). See link below for more details:

https://rclutz.wordpress.com/2020/01/08/heres-looking-at-you...

Now I don't know how the mechanics work. I don't know if you could genetically engineer a C3 plant to work as well as a C4 plant. I reckon there are significant tradeoffs there that nature already factored for.

What humans consider optimal for agriculture and what is optimal in the wild can be different. And evolution is not a perfect optimizer, sometimes it has to maintain legacy structures because what it cannot do is huge refactorings, only incremental changes.

The article also mentions RuBisCO being partially synthesized from chloroplast DNA, which limits evolutionary speed.

Nature does evolve improvements (e.g. better rubisco in microorganisms, the C4 pathway in some higher plants) but other plants can't just download the updates from a central DNA package library.

The pressure on photosynthesis is likely pretty small because nutrients other than carbon dioxide are a limiting factor for almost all plants.

If the current solution is unstable and relatively far away from other solutions, you shouldn't necessarily expect random perturbations to lead to a different solution.