Archive for the ‘anthropoid apes’ Category
discovering the disappearing simian sulcus in humans
dropping off the perch? the lunate or simian sulcus in humans and chimps, FWIW
As an amateur everything, I’ve decided to be more upfront about my amateurishness instead of hiding behind those fictional gropers after truth, Canto and Jacinta, who’ve become sick of being manipulated little poppets. So for the time being I’m going to focus on aspects of evolution as that’s where my reading is currently taking me.
I’ve noticed on my shelves a book, The antecedents of man, by W E Le Gros Clark, FRS, first published in 1959, this second revised edition published in 1962. I don’t know how it came into my possession, I certainly didn’t buy, borrow or steal it (though the third might be the most likely option, but that would’ve been in another life). Mr Le Gros Clark, FRS (aka Sir Wilfrid, bien entendu) was clearly no low-class plodder, and has deserved a wee Wikipedia bio. He was a professor of anatomy at Oxford, but his chief interest was evolution, on which he published many papers (and at least one book, unmentioned in Wikipedia). He was one of the three scholars who exposed the Piltdown Man hoax in 1953.
I’m finding the book, thus far, complex but intelligible, and stimulating. And so to the simian sulcus, something new to me:
In the brains of apes and monkeys there is a very distinctive folding of the cerebral cortex which is often called the simian sulcus. A superficial study might suggest that this is a specialised trait peculiar to apes and monkeys, and one that has been avoided in the evolutionary development of the human brain (which in this particular feature, therefore, would appear to be less specialised). In fact, however, traces of the sulcus are commonly to be found also in the human brain (though they are extremely variable – a characteristic feature of degenerative structures in general), and in the brains of some human races it may on occasion be almost as well developed as it is in the anthropoid apes. It can be inferred, therefore, that the simian sulcus is a primitive element of the brain of the higher Primates and that it has undergone a secondary retrogression in the modern human brain.
So far, so complicated, but I noticed the term ‘some human races’ and their similarity, as regards this feature, to the anthropoid (human-like) apes. Of course, humans are apes, so Clark (to bring the name down a notch) presumably means our ape cousins and ancestors. Race, however, is now regarded as a ‘folk’ category, abandoned by the scientific community.
So what do we now think or know about the simian sulcus? Also known as the lunate sulcus, it’s a ‘fissure in the occipital lobe’, that’s to say, the lobe at the back of the brain, primarily associated with vision. It’s one of the four major lobes of the cerebral cortex, the others being the frontal, temporal and parietal lobes. The lunate sulcus is ‘variably found’ in humans (it’s generally larger in other apes and monkeys) and has been ‘pushed back’ due to the shift in the V1 area, the primary visual cortex or Brodmann area 17. This in turn may have been pushed back to allow for the expansion of the brain’s language regions in the prefrontal cortex and parietal lobes. Let’s face it, the prefrontal cortex is where the human action has been in the pst several millennia.
So, continuing with definitions, what’s a sulcus and what does lunate mean? The cerebral cortex consists of folded grey matter, and the material at the top of the fold is called the gyri, while the grooves at the bottom are called sulci. The term lunate means ‘crescent-shaped’ and there are questions as to whether a lunate sulcus even exists in modern humans, as it has been in a sense obliterated by the relatively rapid expansion and development of the cerebral cortex, a feature unique to the human species. This expansion may have occurred to the slight detriment of the visual cortex in the occipital lobe (actually a pair of lobes separated by a cerebral fissure). So, basically, the lunate sulcus seems to have become increasingly difficult to differentiate from other sulci, and its existence and/or significance is a matter of individual variation in the no doubt highly variable brains of modern humans, not forgetting that said brain is the most complex organ in the known universe (which is but an infinitesimal fraction of the unknown one).
All very fascinating, and I just want to live forever….
References
W E Le Gros Clark, The antecedents of man, 2nd edition, 1962