Metagenomic sequencing: The field exploded after technologies and techniques were developed for using next-gen sequencing to characterize entire populations/communities of living things, first with 16S rRNA sequences, then with full genomes. The cost to do this has also gone down many, many, many orders of magnitude in the last decade or two (just search "sequencing cost graph" on google).

There's been way too many papers. Here [1] is a link to Google Scholar for 'brain gut' which turns up a zillion results. Basically, your gut biome and brain are heavily linked. As a bonus, here's [2] a search for 'glyphosate gut microbiota.' Advances in biotech/consumables and dramatic unforeseen consequences seem completely inseparable.

[1] - https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=brai...

[2] - https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=glyp...

The most cited study on the brain-gut axis going by Google Scholar results was a review paper published in 2011.

My best hypothesis is this review paper inspired the community to investigate the weird relationship.

https://www.frontiersin.org/articles/10.3389/fphys.2011.0009...

There have been thousands of breakthroughs in the field. The pace of research and progress is incredible.