At least, it hasn’t been seen in other life forms. From ScienceDaily:
Tamás and University of Szeged doctoral student Eszter Boldog dubbed these new cells “rosehip neurons” — to them, the dense bundle each brain cell’s axon forms around the cell’s center looks just like a rose after it has shed its petals, he said. The newly discovered cells belong to a class of neurons known as inhibitory neurons, which put the brakes on the activity of other neurons in the brain.
The study hasn’t proven that this special brain cell is unique to humans. But the fact that the special neuron doesn’t exist in rodents is intriguing, adding these cells to a very short list of specialized neurons that may exist only in humans or only in primate brains.
The researchers don’t yet understand what these cells might be doing in the human brain, but their absence in the mouse points to how difficult it is to model human brain diseases in laboratory animals, Tamás said. One of his laboratory team’s immediate next steps is to look for rosehip neurons in postmortem brain samples from people with neuropsychiatric disorders to see if these specialized cells might be altered in human disease. Paper. (paywall) – Eszter Boldog, Trygve E. Bakken, Rebecca D. Hodge, Mark Novotny, Brian D. Aevermann, Judith Baka, Sándor Bordé, Jennie L. Close, Francisco Diez-Fuertes, Song-Lin Ding, Nóra Faragó, Ágnes K. Kocsis, Balázs Kovács, Zoe Maltzer, Jamison M. McCorrison, Jeremy A. Miller, Gábor Molnár, Gáspár Oláh, Attila Ozsvár, Márton Rózsa, Soraya I. Shehata, Kimberly A. Smith, Susan M. Sunkin, Danny N. Tran, Pratap Venepally, Abby Wall, László G. Puskás, Pál Barzó, Frank J. Steemers, Nicholas J. Schork, Richard H. Scheuermann, Roger S. Lasken, Ed S. Lein, Gábor Tamás. Transcriptomic and morphophysiological evidence for a specialized human cortical GABAergic cell type. Nature Neuroscience, 2018; DOI: 10.1038/s41593-018-0205-2 More.
As to the question of whether the cells contribute to human bran uniqueness?
A few other entries on the question of how the human brain came to be what it is:
Some say we evolved large brains alongside small guts, but another research team found no such correlation. Alternatively, fluid societies (relative to chimps) explains it. And, according to some, mental illness helped. Chimpanzees’ improved skills throwing excrement are also said to provide hints about human brain development. (The ability to throw projectiles at very high speeds is apparently unique to humans.) Our ancestors had to grow bigger brains anyway, we are told, to make axes and hunt something besides elephants. Collective intelligence (“ideas having sex”), whatever that means, has been really important to human evolution as well. More.
See also: Human evolution researchers: Social challenges decreased brain size
Did large brains cause Neanderthals to go extinct?
Homo naledi’s small but sophisticated brain challenges belief in “an inevitable march towards bigger, more complex brains.”
Human origins: The war of trivial explanations