Friday 19 January 2024
You should listen to your gut?
New Findings About Our Mysterious “Second Brain”
It wasn’t long ago that researchers were hardly aware of the way the digestive system functions as a second brain. The big focus was neurons. But, along with neurons, both the central nervous system and the digestive system make extensive use of glial cells, whose function has not been as well understood.
Glial cells, which do not produce electrical impulses, were considered “electrophysiologically boring.” We now know that they support neurons in both physical and chemical ways. In the gut, they co-ordinate immune responses. From the Francis Crick Institute, we learn:
… the enteric nervous system is remarkably independent: Intestines could carry out many of their regular duties even if they somehow became disconnected from the central nervous system. And the number of specialized nervous system cells, namely neurons and glia, that live in a person’s gut is roughly equivalent to the number found in a cat’s brain.
MOHAMMAD M. AHMADZAI, LUISA SEGUELLA, BRIAN D. GULBRANSEN. CIRCUIT-SPECIFIC ENTERIC GLIA REGULATE INTESTINAL MOTOR NEUROCIRCUITS. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES, 2021; 118 (40): E2025938118 DOI: 10.1073/PNAS.2025938118 THE PAPER IS OPEN ACCESS.
Researcher Brian D. Gulbransen explains, “In computing language, the glia would be the logic gates. Or, for a more musical metaphor, the glia aren’t carrying the notes played on an electric guitar, they’re the pedals and amplifiers modulating the tone and volume of those notes.”
Understanding how much the digestive system functions, in part, as its own brain may help researchers develop better treatment for the gut disorders that afflict about 60 to 70 million people in the United States alone.
The Role Microbes Play
The vagus nerve is a stout cable of neurons that serves as an information highway between the base of the brain and the gut. Even though it is the longest nervous system connection in the body, messages take only milliseconds to travel between the brain and the gut.
The really surprising thing is that the trillions of microbes that inhabit a human digestive system play a role in all these communications, as University of British Columbia neuroscientist Heather Gerrie notes:
Many of these microbes live in the mucus layer that lines the intestines, placing them in direct contact with nerve and immune cells, which are the major information gathering systems of our bodies. This location also primes microbes to listen in as the brain signals stress, anxiety or even happiness along the vagus nerve.
But the microbes in our gut microbiome don’t just listen. These cells produce modulating signals that send information back up to the brain. In fact, 90% of the neurons in the vagus nerve are actually carrying information from the gut to the brain, not the other way around. This means the signals generated in the gut can massively influence the brain.
HEATHER GERRIE, “OUR SECOND BRAIN: MORE THAN A GUT FEELING,” UNIVERSITY OF BRITISH COLUMBIA GRADUATE PROGRAM IN NEUROSCIENCE.
A Constant Battle
Another of the remarkable qualities of glial cells is that they can shift from one type to another, as needed, in the constant battle to keep pathogens and toxins at bay.
As Yasemin Saplakoglu points out at Wired,
… scientists now know that enteric glia are among the first responders to injury or inflammation in gut tissue. They help maintain the gut’s barrier to keep toxins out. They mediate the contractions of the gut that allow food to flow through the digestive tract. Glia regulate stem cells in the gut’s outer layer, and are critical for tissue regeneration. They chat with the microbiome, neurons, and immune-system cells, managing and coordinating their functions.
YASEMIN SAPLAKOGLU, “UNPICKING THE MYSTERY OF THE BODY’S ‘SECOND BRAIN,’” WIRED, JANUARY 14, 2024
This “chat” among neurons, glia, and microbes could be important for research into the digestive system in relation to mood disorders and anxiety and depression. People often assume that their stomachs are upset because they are emotionally upset. But the story of the millions of communications shunted back and forth in milliseconds could be more complicated than that.
As Jay Pasricha, M.D., director of the Johns Hopkins Center for Neurogastroenterology, says,“The enteric nervous system doesn’t seem capable of thought as we know it, but it communicates back and forth with our big brain — with profound results.”
Homo Habilis is the missing link?
Fossil Friday: New Research Questions the Human Nature of Homo habilis
Last year, I wrote an article (Bechly 2023) for Fossil Friday about the questionable status of the East African fossil hominin Homo habilis as a member of our genus rather than being just another ape-like australopithecine. This is a crucial issue for human origins, because Homo habilis is often proclaimed as the transitional form connecting our genus with australopithecines. As I elaborated in my article, this notion was challenged by several mainstream experts, and it was challenged from quite early on (e.g., Wood 1987). But this challenge is far from being a thing of the past. Two new articles relevant to the status of Homo habilis appeared last year in the 50th anniversary edition of the Journal of Human Evolution.
The first article was authored by Bruner & Beaudet (2023), who reviewed the evidence from three decades of research on the brain of Homo habilis. They found that “after more than 30 years, the fossil record associated with this taxon has not grown that much” and concluded that “in this sense, the disciplines working with fossils (and, in particular, with brain evolution) should take particular care to maintain a healthy professional situation, avoiding an excess of speculation and overstatement.” In other words, based on our current knowledge of the fossil record and the human brain, the scientists did not find compelling the claims of Tobias (1987) who had originally “suggested that the neuroanatomy of this species evidenced a clear change toward many cerebral traits associated with our genus.”
“Excessive Speculations”
It is also quite interesting that the lead author commented on his blog site (Bruner 2022):
This taxon [Homo habilis], much debated in the last 20 years, has not found a proper taxonomic validation yet, which suggests at least a lack of robust evidence, in this sense. … The attention of the mass media for science and research is prompting a compulsive marketing based on appearance and fast vending news, at the expense of content and quality. Paleontological fields are characterized by issues that can be hardly proven, charming topics, and harmless conclusions (in the sense that they have no direct consequences on people’s welfare). These three features make these fields more sensitive to contamination associated with personal, institutional, and economic interests, generating a conflict between scientific proficiency and public visibility. Excessive speculations, in this sense, can seriously harm the reputation of the discipline.
Bruner also commented on the quite disturbing impact of woke cancel culture on his submitted review of three decades of paleoneurology, which should also include two photos of the founders of the discipline, namely the distinguished scientists Phillip Tobias and Ralph Holloway:
After one year of preparation, three resubmissions, and the revision performed by four referees, the publication was suddenly suspended during the proofs’ correction stage, because it included these two images of “white senior males”, which apparently goes against the defense of human diversity (race, age, and sex). When the Editors were informed of this situation, they circumvented the problem, saying that an informal and unwritten norm of the journal prevents the possibility of publishing photographs of persons, except for obituaries. A strange norm, which apparently undervalues the fact that science is done, inevitably, by persons. … Our review ends by asking what we should keep from 30 years of paleoneurology. More than fossils and techniques, what really matters is, after all, competence, expertise, experience, and commitment. These four values, apparently positive for the sake of a healthy science, were also criticized during the proofs’ correction (most of all the term “commitment”, which had to be substituted in the final version of the manuscript), because they were claimed of having a negative connotation, supporting “meritocracy”.
Unfortunately, like the rest of Western society, modern science seems to be going more and more bananas.
A Jaw-Dropping Conclusion
Anyway, the second article was published by Antón & Middleton (2023), who re-evaluated the fossil record of early Homo, especially H. erectus, H. habilis, and H. rudolfensis. Their conclusion is jaw-dropping: “Chronologically and morphologically H. erectus is a member of early Homo, not a temporally more recent species necessarily evolved from either H. habilis or H. rudolfensis”. So much for the latter two taxa as missing links between ape-like australopithecines and real humans of the genus Homo. Homo erectus coexisted with Australopithecus and tools associated with “Homo” habilis may have been used by Homo erectus on Australopithecus habilis rather than being produced and used by the latter, which is suggested by the distribution of fossils and artifacts at the Olduvai gorge site in Tanzania (Bechly 2023). Habilis was likely not a handy man but the ape bush meat of real humans.
This is not just my humble opinion, but is also supported by a brand-new study by Davies et al. (2024) in Nature Communications on the dental morphology of Homo habilis. The authors found this “morphology in H. habilis is for the most part remarkably primitive, supporting the hypothesis that the H. habilis hypodigm has more in common with Australopithecus than later Homo.”
Not About Consensus
Thus, the most recent research confirms the previous and early critique that H. habilis should be classified as Australopithecus. So, the latter view cannot be dismissed as obsolete and outdated science, and it can also not be dismissed by a mere appeal to a scientific consensus. Science is not about consensus but about empirical evidence and rational arguments. A consensus is scientifically worthless when it is driven by worldview bias and peer pressure rather than by an unbiased inference to the best explanation. Here is what Jurassic Park author Michael Crichton famously said about this issue: “I regard consensus science as an extremely pernicious development that ought to be stopped cold in its tracks. Historically, the claim of consensus has been the first refuge of scoundrels; it is a way to avoid debate by claiming that the matter is already settled.”
References
Antón SC & Middleton ER 2023. Making meaning from fragmentary fossils: Early Homo in the Early to early Middle Pleistocene. Journal of Human Evolution 179(S66): 103307. DOI: https://doi.org/10.1016/j.jhevol.2022.103307
Bechly G 2023. Fossil Friday: To Be or Not to be Homo. Evolution News June 23, 2023. https://evolutionnews.org/2023/06/fossil-friday-to-be-or-not-to-be-homo/
Bruner E 2022. Three decades of paleoneurology. paleoneurology November 28, 2022. https://paleoneurology.wordpress.com/2022/11/28/three-decades-of-paleoneurology/
Bruner E & Beaudet A 2023. The brain of Homo habilis: Three decades of paleoneurology. Journal of Human Evolution 174: 103281. DOI: https://doi.org/10.1016/j.jhevol.2022.103281
Davies TW, Gunz P, Spoor F, Alemseged Z, Gidna A, Hublin J-J, Kimbel WH, Kullmer O, Plummer WP, Zanolli C & Skinner MM 2024 Dental morphology in Homo habilis and its implications for the evolution of early Homo. Nature Communications 15: 286, 1–16. DOI: https://doi.org/10.1038/s41467-023-44375-9
Tobias P 1987. The Brain of Homo habilis: A New Level Of Organization in Cerebral Evolution. Journal of Human Evolution 16, 741–761. DOI: https://doi.org/10.1016/0047-2484(87)90022-4
Wood B 1987. Who is the ‘real’ Homo habilis? Nature 327, 187–188. DOI: https://doi.org/10.1038/327187a0
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