Image from NOAA, in the public domain.
Last updated on March 16, 2018. I just finished a large project trying to estimate that. I’ve posted it on its own page here. Here’s the abstract:
We estimate that there are between 10^23 and 10^24 neurons on earth. Most of this is distributed roughly evenly among small land arthropods, fish, and nematodes, or possibly dominated by nematodes with the other two as significant contenders. For land arthropods, we multiplied the apparent number of animals on earth by mostly springtail-sized animals, with some small percentage being from larger insects modeled as fruit flies. For nematodes, we looked at studies that provide an average number of nematodes per square meter of soil or the ocean floor, and multiplied them by the number of neurons in Caenorhabditis elegans, an average-sized nematode. For fish, we used total estimates of ocean fish biomass, attributed some to species caught by humans, and used two different ways of allocating the remaining biomass. Most other classes of animal contribute 10^22 neurons at most, and so are unlikely to change the final analysis. We neglected a few categories that probably aren’t significant, but could conceivably push the estimate up.
Using a similar but less precise process based on evolutionary history and biomass over time, we also estimate that there have been between 10^32 and 10^33 neuron-years of work over the history of life, with around an order of magnitude of uncertainty.