Digitized Fossil Brains “Neocortex” Harry Jerison <http:\\ hjerison. Bol. Ucla.edu> Paleoneurology: Fossil Brains Digitization: laser and other scanning Measurement: surface area, volume Encephalization and Neocorticalization Language (speculation)

This is a fossil “brain,” an endocranial cast from a small late Eocene artiodactyl about 37 million years old. Its “brain” volume is about 10 ml; the animal was about the size of a living cat. You recognize olfactory bulbs, forebrain, and hindbrain on this rock.

Laser scans of the endocast and measurements on this Bathgenys reevesi Laser scans of the endocast and measurements on this Bathgenys reevesi. (Green marked“neocortex” by laser software)

Artist’s drawing of the oreodont Merycoidodon culbertsoni, a relative, presumably a descendant, of a few million years later. Imagine it cat-size.

Paleoneurology (Fossil “Brains”) The slides so far cover the elements of paleoneurology applied to a particular species, Bathygenys reevesi. Endocasts are good enough pictures of brains that one can treat them as the brains of particular animals. They are fossil finds, either from skulls or from natural endocasts like the one I showed you. Digitization converts the physical endocast into a computer image that can be measured. The most important measurement on these fossils is of surface area. Before showing you why this is especially important I must show you some human endocasts, about which you must think a bit harder.

Two human endocasts (laser scan, left, and mri scan, right) Two human endocasts (laser scan, left, and mri scan, right). What’s wrong? Poor image of the Sylvian fissure -- no good data for guesses on language. Next: the living human brain.

Left and right human hemispheres (Wisconsin Brain Collection 69-314) Left and right human hemispheres (Wisconsin Brain Collection 69-314). Note left Sylvian is a bit longer than right. Heschl’s gyrus (“language area”) is buried within posterior left fissure.

3-D Digitizing Laser Scanning You saw a digitized scan before

3-D Digitizing Laser Scanning “Rhinal fissure” is outlined (blue) on scan.

A second example compares the endocast of a late Eocene prosimian Adapis parisiensis with the brain of a living galago Galago senegalensis. These were both about 10 ml in volume. I marked “neocortex” in green on the fossil. You can see a bit of rhinal fissure on galago.

Laser scanning: The scanner is adjacent to my computer monitor, which shows the image of the Adapis endocast on the screen -- “neocortex” is marked green. The picture at the right shows the specimen on the scanner platform. (The disarray of skulls and endocasts is normal in my workplace.)

The “Cyberware”scanner platform with the endocast of Adapis parisiensis in place. Images in the scanner to the left of the platform are of laser-optics prisms.

My scan of the Eocene prosimian Adapis parisiensis is FMNH 59259 My scan of the Eocene prosimian Adapis parisiensis is FMNH 59259. Galago is from <http://brainmuseum.org/Specimens/index.html> Wisconsin (61-686 ), an invaluable resource for comparisons of living brains.

Measurement : Cortical surface area and information processing capacity. This graph is from my James Arthur Lecture on “Brain Size and the Evolution of Mind” (American Museum of Natural History, 1991). A few of its 50 species are named to indicate their great variety. The correlation of 1.00 to two significant figures indicates the strong relationship: brain size is an almost perfect between-species estimator of cortical surface area. Surface area is an excellent estimator (between-species) of the total number of neurons in a brain, hence of processing capacity.

Neocortex and Mind Neuroscientists have generally assumed that mind is a consequence of cortical activity, in particular of neocortex. To determine more about the evolution of neocortex the first issue was to estimate neocortical size in endocasts of fossil mammals. This is surprisingly easy, because the rhinal fissure is a visible boundary line in most endocasts. The uniformitarian hypothesis assumes it is true for fossil as well as living brains. In living mammals neocortex is cortex dorsal to the rhinal fissure as is probably true in fossils.

Rhinal fissure as ventral boundary to neocortx: Armadillo brain and cross-section (Wisc 60-465) showing border of neocortex.  

Rhinal fissure is partly hidden in most primate endocasts Rhinal fissure is partly hidden in most primate endocasts. The slide has human and Adapis at the left, which you have seen before, and an endocast and brain of the mandrill, Papio sphinx. The adapid and monkey are shown ventrolaterally as well as dorsally, but the mandrill brain from the Wisconsin collection is shown only laterally. My green marking of left neocortex is manual and only approximate. Both the human and mandrill neocortex are about 80 % of the brain’s surface area. The Eocene adapid’s is about 65 %. Human endocast 1370 ml; mandrill 132 ml; Eocene adapid, 8.3 ml.

Encephalization and Neocorticalization Encephalization is measured by brain size relative to its expected size. In this example, an early Eocene equoid,“Hyracotherium.” A, rendered endocast. B, tesselations (voxels) for the scan. C, Accurate scale model that had been scanned. Endocast volume is 24.1 ml. Scaling up the model leads to a body size estimate of 10.7 kg. Digitization provides the best guess on body size to estimate encephalization.

Encephalization in Amniotes (Next slide places living and fossil horses in the mammal convex polygon.)

(T is a condylarth endocast misidentified as an equoid.) Encephalization of “Hyracotherium” 1 (points are living and fossil equids) “Hyracotherium” is point 1 at left. Dashed line is average living mammal allometry: Y =.05 X.74. Solid line: Y = 0.12 X2/3. (T is a condylarth endocast misidentified as an equoid.) Digitization is not a major contribution for encephalization, which can be estimated from routine brain and body weights determined as in the past. Digitization is the only way to measure neocorticalization on the irregular surface of a brain or endocast. It is the ratio of surface area of neocortex to total surface area of the brain or endocast.

Neocorticalization To return to an early slide in which the measurements were shown, the idea is to measure total surface area and then the total neocortical area. For technical reasons I eliminate olfactory bulb area from total endocast area. The neocorticalization ratio is 9.4/ 31.5 = 30%. This is done for all of my 150 or so specimens. A preliminary graph shows the evidence clearly on the next slide.

Preliminary Result on neocorticalization Neocorticalization in Mammals There are three Eocene prosimians and two Plio-Pleistocene australopithecines in this sample. Living primates included two humans and a mangabey at the same maximum neocorticalization. The main result is that taxa differed. Primates have always been more neocorticalized than other mammals, marsupials always less. Over the 60 million year span there was an average increase of neocorticalization at about 5% per 10 million years.

There may be an error in the ordinate in the previous (preliminary) slide. Here is a later graph with more species.

DISCUSSION: Why did neocorticalization increase (even though it's only 5% per 10 million years). There was evidently some selective advantage to more neocortex. How does language fit in? In addition to its role in communication, language is a uniquely human cognitive trait. Years ago Joe Bogen, who did the cutting that showed the left brain and right brain as different, told me that as a neurosurgeon he was especially careful about cutting in ventral temporal lobe. It was that region rather than Heschl's gyrus or Wernicke's area that worried him the most. Removing it had terrible effects on language. Our view of the precise localization of language is part of our general disposition to find localized centers. Language certainly has these, but like other cognitive controls it is dispersed through much of the neocortex. Enlarged neocortex occurs to the same proportional change in primates. It's only our very large brain, the absolute size of our proportion, that is so remarkable. I would not look for specific areas in the fossil record of the hominine brain for a localized language area. The increase in the hominine neocortex really explains the changing cognitive capacities.

Thanks I have too many colleagues to thank for access to their collections. The Field Museum of Natural History in Chicago and the late Len Radinsky, who left his collection of endocasts to the Museum, deserve special thanks. The Cyberware Corporation in California provided the apparatus that supports my scanning.