We are not accustomed to thinking of fruit flies as even having brains. But they have 120 types of neurons in their visual system alone (which could be why they are so pesky):

In the research published in Nature, the researchers studied the brains of the fruit fly Drosophila to uncover the complete set of tTFs needed to generate the roughly 120 neuron types of the medulla, a specific brain structure in the visual system of flies. They used state-of-the-art single-cell mRNA sequencing to obtain the transcriptome — all of the genes expressed in a given cell — of more than 50,000 individual cells that were then grouped into most of the cell types present in the developing medulla…

The researchers then identified the genetic interactions that allow the temporal cascade to progress and how this progression relates to the “birth order” of all neurons in the medulla, linking specific temporal windows with the generation of specific types of neurons. This cascade is necessary to produce the full extent of neural diversity of this brain region in a stereotypic order.

New York University, “Scientists pinpoint what makes brain cells develop in a specific order” at ScienceDaily (April 6, 2022) The paper requires a fee or subscription.

The researchers think that human brains probably unfold in a similar serial order of neuron types:

Finally, the team examined the first steps in the process of neural stem cells maturing into neurons, a stage in neuron development called differentiation. They found that the differentiation process for fly neurons and human cortical neurons was remarkably alike, with similar patterns of genes expressed during the various stages of differentiation.

New York University, “Scientists pinpoint what makes brain cells develop in a specific order” at ScienceDaily (April 6, 2022) The paper requires a fee or subscription.

The researchers hope that by identifying the stages at which neurons are either missing or go wrong, they can possibly develop better treatments to insert or replace them.

Note: The remarkable fact is that human and fruit fly brains are so similar but with vastly different results. Clearly, the brain is not all we need to know about a life form.

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