Even if it had evolved, any detailed form of communication that had the potential to transmit hard-to-break imperatives is something you want to be very, very careful with.

Defection, manipulation, and novel avenues for disease-transmission or parasitism heavily disincentivize this. It's intuitively "gross" for a reason.

TL;DR: Infections and defections would probably utterly wreck this. The blood-brain barrier exists for a reason. While we did get language, in other ways we've evolved specifically to hide information from each other; it's not straightforwardly evolutionarily favored. Large clusters of highly-related organisms have more incentive to do this (bacterial mats, ants, our own cells, etc.), and the information-bandwidth they share with each other through pheromones and chemical signals is actually pretty staggering. But at a glance, I do think they pay a cost in increased (and more elaborate) avenues for manipulations and infections to reap the benefits of this privilege.

Edit to add: Linking some additional strongly-related articles! SSC's Maybe Your Zoloft Stopped Working Because a Liver Fluke Tried To Turn Your Nth-Great-Grandmother into a Zombie and the paper it centers on, Invisible Designers: Brain Evolution Through the Lens of Parasite Manipulation

Isolating Brains from Infection

It bears pointing out that evolution usually seems to want the opposite of this for our centralized-decision-maker organs. Notice that instead of making the brain more open-access over time, most species went the direction of making it isolated from even our own bodies, using things like the heavy-filtering brain/blood barrier. The risk of that sensitive organ getting poisoned, diseased, or biologically manipulated was just too high to risk it.

(Nematodes make what humans call "embodied thinking" look like a joke; the serotonin from their digestive tract is felt by their brains directly.)

People used to die in droves (and at young ages!) of measles, tuberculosis, and a million other things. Even before cities, herd-living put us under immense pressure to develop a pretty intensely specialized immune system. If we had to worry about giving a disease a highway to our central nervous system, that would be... very, very bad. It might even guarantee that such a species would never get to centralize such things at all, such a force is the risk of infection.

Not to even get into the possibility of physical brain-hijackings by concepts (like memes, but oh-so-much-worse!), or even just catching communication-transmitted Kuru... but here's a pretty vivid speculative description of just how bad being an evolved "open book" that granted others write-access would probably get.

And with regards to the "benefits" of open communication -of information conveyed in a language that's very hard to fake- we do still have some information transmitted in body-language and words. That certainly captures some of the benefit. But it bears pointing out that we're a species that un-evolved any obvious presentation of whether a female is in estrus, and has very strong inhibitions around trying not to gain information from each other's body odor. "Complete, total honesty" is not something evolution typically selects for, and it didn't veer entirely that way for us. Even in the less-cutthroat modern era (at least, compared to our distant savannah past), Greg Egan's Closer feels like a pretty realistic depiction of how we might feel about it if we ever did fully share our mental experience with even one another person. I'll avoid spoiling it too badly, but we'd probably quickly uncover a lot of things about one other that we wished we didn't know.

Some Living Approximates

Bacterial mats, giant networks of fungi, and eusocial insects with strong genetic kinship might have strong enough evolutionary incentives for this to line up, although higher relatedness actually exacerbates the infection concern. And between the cells of multicellular creatues, certainly quite a lot does get communicated. Many of these examples do seem to "share their mind" in at least some meaningful sense. They transmit a lot of information and orders to one another, and have a communal decision-making process at varying degrees of centralization/decentralization. Pheromones for insects, various signalling secretions from bacteria, the oodles of transactions and deliveries between our cells at every moment... combined these can be very high-bandwidth. Almost incomprehensibly so, if you've ever seen attempts to measure and chart such things.

Ants could practically be said to have a pheromone "language," complete with clan-identification tags. And as a way to selectively trigger a highly-specific neural pathway, or activate a known set of behaviors in a conspecific with the same brain-configuration, pheromones are not a bad way to go? The behavior patterns pheromones set off can get oddly specific at times.

And... ants also get tricked by pheromones into feeding the brood parasites that eat their own young. And what we call "bacterial sex" (high-bandwidth communication of DNA?) is actually virus-esque plasmids trying to transmit themselves to new bacteria, like an infection. Some plasmids might even come with addiction molecules, which is an extra-douchey way for an plasmid to convey "replicate me, or die." And in coordinated bacteria, you do sometimes see defectors. So... it's still pretty manipulable, and it sure gets manipulated.

The more stereotyped behaviors you can set off through external signals, the more you have a "broader attack surface," in the cybersecurity lingo. And biological parasitism is ubiquitous, and fractaline, and adaptive, and uses any damn attack surface it can get.

Humans? A fluke. Parasites are evolution's true darlings.

There are lots of simple things that organisms could do to make them wildly more successful. The success of human society is a good demonstration of how very low complexity systems and behaviours can drive your competition extinct, magnify available resources, and more, the vast majority of which could be easily coded into the genome in principle.

However, evolution does not make judgements about the end result. The question is whether there is a path of high success leading to your desired result. Laryngeal nerves are a good demonstration that even basic impediments won't be worked around if you can't get there step by step with appropriate evolutionary pressure. Ultimately there seems to be no impetus for a half-baked neuron tentacle, and a lot of cost and risk, so that will probably never be the path to such organisms.

There are many examples of fairly direct inter-organism communication, like RNA transfer between organisms, and to the extent that cells think in chemicals, the fact they share their chemical environment readily is a form of this kind of communication. I'm not aware of anything similarly direct at larger scales, between neurons.

currently the most advanced form of communication to have evolved, human language, is still very low bandwidth

Language is just one of the means of communication available to human beings. Its seemingly low bandwidth is due to the fact that the neocortex, the part of the brain responsible for processing language, logic, visual reasoning etc, is itself rather low bandwidth/low speed, so language itself isn't the bottleneck. (The bottleneck itself is mainly due to the more recent evolution of this neocortex, and the fact that these processes were not that vital to human survival until very recently on an evolutionary timescale. Also that these logical processes take up a lot more energy than the some of the other processes happening in the brain.)

But human nervous systems do have much higher bandwidth communication channels. We share them with the other mammals. It's the limbic system, that based on the tone of another person's voice, his posture, his facial muscles and a lot of other indicators, leads to a split second reaction, a gut feeling, an instinct about another person, his standing/status in the herd, etc being formed.

Going a level lower, there is an even faster, more primitive system, the basal ganglia, or the so called reptilian brain, this is where sensory input is first processed. For instance, in a split second you can tell if some set of input represents a dangerous situation, leading to a fear response.

In complex vertebrates it is the amygdala that first processes these sensory inputs, matches them against a fast type of emotional memory, and triggers the appropriate response through connections to the hypothalamus (for instance, triggering the flight-flight-freeze response).

In any case, I think the question itself is a bit naive on the brain architecture side from an evolutionary perspective. The nervous systems of these mammals do process and communicate a great deal of information (of course, this being an evolutionary process, there are things other than the brain and communication competing for energy). In the case of humans, most of that communication is not conscious. The part that is conscious is just a very recent addition, and was not that vital until very recently.

Communication is only possible when you agree on the meaning of signs in the messages you exchange. You need human language regardless whether you have neural i/o channels or use sound waves to communicate.

While some humans do use gestures while they communicate to add more bandwidth there seems to be little pressure to go for more bandwidth by evolving a system of more elaborate gestures when you are able to speak with another person.

The part of language that's about thinking to how abstract words relate to meaning seems to be more import than adding more i/o.

Direct neural IO has a large fitness moat. Once an animal has any kind of actuator that can modify the environment, and any kind of sensor that can detect info about the environment, then one animals actions will sometimes modify what another animal senses, and hence how it behaves. Evolution can then get to work optimizing this. Many benefits can accrue, even if no other animal communicates. A crow pattering its feet to bring up worms has some understanding of other animals being things it can manipulate, and the tools to do it. (humans are best at training other animals as well as communicating, both need a theory of mind.)

Animals don't touch neurons together except in freak accidents, where any chance of survival is minimal. Until you have functional communication, banging neurons together is useless. Until you have a system that filters it out, saline exposure will spam nonsense. And once you have one form of communication, the pressure to develop a second is almost none.

I'm surprised nobody has mentioned risks of contagion and attack. Internal neural links are subject to viral and autoimmune problems, but there's at least a commonality of DNA on both ends of the link, so a shared evolutionary fate and at least a basis for trust.

Anyone who's done any infosec or network protocol work will laugh at the idea that trial and error (evolution) can make a safe high-bandwidth connection.

While I am also not sure I should try answering here, seems that an obvious first cut at answering the first question would that to address why it would be an evolutionary advantage? How good does the communication need to be for survival and being the fittest for whatever niche is to be occupied?

Behind that might be two related questions/assumptions. One, when a mutation evolves to provide and advantage in some environment should we expect big changes, little changes or some ongoing sequence of "improvements" leading to some big change (think rapid changes over a relatively short period). Putting that a bit differently, is evolution a slacker that settles for the minimum or some type of maximizing process?

The other question/assumption is what are the constraints or opportunity costs for an evolution. Just how big a change can occur as one might expect that to be a costly process for the organism.

[And yes I realize the above sounds a bit like evolution as a conscious or intelligent process but that is due of writing quickly and so poorly expressing the thought ...]

One of the reasons might be that one human is a individual information processing system. You need feedback loops to have context for your sensory data. If you link two brains by a magic interface there is the issue that the remote brain doesn't have any context what the data from the local eye means. Two individuals might also be using totally different framworks for incorporating knowledge. Someone might be more visual based, someone more kinetic. Now if you transfer a kinetic encoded knowledge to a brain that uses visual representations the data is just structure incompatible with the receiver and unable to be used for any purpose. You have to actually translate the data to information types that can actually be utilised by the brain and this work can't be skipped by arbitrary fast data transfer ie communication is not a substitute for understanding.

A species that used a standardised locked-in information scheme could do such tranfers as any data generatred in any brain would be usable verbatim in other brains. But a species that allows individual organism run on different operating systems can discover new types of operating systems, tailor operating system to environment and benefit from commuinities that have information trade between different types of OSes. And while lossless perfect transfer might be powerful, dialogue between two archtechture migth be even better. Instead of losses you get gains. Settling at 100% might mean leaving money at the table.

Part of the miracle of language might be that you get interoperability with full implementation freedom. And note that there is nothing stopping you from intaking information on the various human languages. But the chances are you only undersand 1 or couple of them and if an expert somewhere near a dozen. Most languages have dialects ie they are at the risdk of breaking into even small languages. In the limit world where every human speaks their own language thoughts would travel very slowly. But all islands of interoperability need energy upkeep to keep that interoperability going. Silence is the default, sharing requires effort.

Communication requires both input and output channels. All of the instances I can think of from the animal world involve a sense (hearing, sight, smell, touch) which has evolved with a different benefit. Then an output can evolve to take communicate using this sense as the input.

This seems orders of magnitude less complex than evolving input and output simultaneously which would be required for direct brain communication (a least I can't think of another option).

Even if it could potentially happen, before it did there would be many instances of indirect communication evolving. Take-off happening first in a species with indirect communication is a fairly inevitable consequence of the relative complexity of the evolutions required.

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