Why did the evolution of a large brain happen only once (among mammals, and in particular, primates?)
Larger brains have evolved a number of times. It seems that there has been a trend over several tens of millions of years of evolution of larger brains in various clades, such as carnivores and primates. There is probably a kind of arms race going on among various species in which a larger brain is an asset.
However, as you imply, a really large brain (like the extraordinarily large human brain) seems to be very rare. One of the reasons for this is that there are at least two major kinds of costs of a large brain that outweigh the benefits. One kind of cost is the energetic expense of having this large brain. Over 10% of the day to day energy demands of an adult human go to the brain. The total energy requirement of an infant can be over 60% while the brain is both a relatively large proportion of the infant's body, and is undergoing a great deal of growth. The brain tissue is very picky about things like the temperature it requires for normal function and the kind of nutrient it needs.
The other negative, which is also a positive, is that behavior mediated by cerebral function (roughly, "thinking") requires learning. Organisms with larger brains are not born "knowing" what to do. This is a benefit because it allows for more flexibility in behavior and it allows for kinds of behaviors that probably can't be "programmed" genetically. The down side is that a lot of learning needs to happen, and it has to happen in the right way, or you get a dysfunctional individual.
The challenge of behavioral biologists interested in humans is to ascertain how the costs and benefits of a large brain interact with the environment of adaptation in which we see this large brain emerge over evolutionary time.
How many species of early hominid existed at any one point in time?
Sahelanthropus tchadensis, Orrorin tugenensis, Ardipithecus ramidus, Australopithecus anamensis, Australopithecus afarensis, Kenyanthropus platyops, Australopithecus africanus, Australopithecus garhi, Australopithecus aethiopicus, Australopithecus robustus, Australopithecus boisei, Homo habilis, and Homo georgicus is a short list of species that lived from about 6.5 or so million years ago to about 1.8 million years ago. Many of them lived during the "golden age of the hominid" between about 4 and 2 million years ago. There are others not listed here that are being or have recently been proposed.
With some clades of primates, we know that the species differences are seen entirely or almost entirely in the non-skeletal parts. This suggests that this number of species is a severe underestimate of the actual numbers. We also know that many primates have moderate size or small ranges, and this list comes from only a handful of localities across the region in which hominids lived, so there must be more species living in areas not sampled by the fossil sites. And, the rate of discovery of new species has not slowed down, or at least, it is reasonable to say that we can expect that the more that we look, the more we find, for the foreseeable future.
So the prospect that several, perhaps quite a large number, of hominid species would have existed at one point in time across most of Africa and possibly west Asia, is a near certainty. You would need a guidebook to figure out what you were looking at if you went back in a time machine to this region.
How many of them interacted and what was this interaction like?
Another common feature of primates is that many social primates do get into inter-species groups. There are various advantages to this. As long as food competition is minimal, mixed species groups may form to decrease predator pressure, for instance.
My personal feeling is that there was likely a fair amount of interaction between different hominid species. One reason for this is that not long after the first appearance of chipped stone tools, we see them in the record over a fairly large area. My guess (and it is only a guess at this point) is that stone tool use is more widespread at, say, 2.2 million years ago than it could have been if it was only practiced by one species. Interaction among species would facilitate the spread of this technology. That is only a guess, of course, and there are a lot of other ways to look at the data.
What caused those that disappeared to do so?
Perhaps there was a mass extinction related to increaced drying of the environment between 2 and 1.5 million years ago. Perhaps the rise of Homo erectus led to all of the others being outcompeted (or eaten for food!?) I find it interesting that relatively little is written about this. This may be in part because the time period we need to understand has fewer well dated deposits with fossil than we would like to address such questions.
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I hear the Kenyanthropus platyops was quite tasty. Mmmmm barbecued hominid...
Yeah, but Australopithecus robustus has a bold flavor that goes well with a nice red wine.
IMO you left out the most important negative re brain size: Infant head size. Humans have carried two adaptations for large brains to an extreme extent:
1) The size of the human infant head is so large that it creates a risk during childbirth for both mother and child.
2) The human infant is born extremely premature in brain development compared to other primates (including its closest relatives). This requires an extended period of intensive maternal care not needed by those relatives, with concomitant disadvantages for reproduction.
I can probably scare up references for the above if you require, but I suspect you have them at your fingertips, and I don't.
Infant head size is very important. It relates not so much to the question of encephalization as to limiting encephalization, but it is important.
The uniqueness of hyperencephalization demonstrates the low survival value of a slightly larger brain. Our overpopulation suggests there's (at least temporary) value in having a fully three-pound brain. In between, something unusual had to happen to push us over the hump. Darwin suggested sexual selection, but that it didn't happen in other species means something else was at work (too).
Colonization by the language meme, and co-evolving with it, might be the something else, but it must be a later development. Some have suggested that throwing rocks was a driving force, because to double accuracy takes four times as much brain. An animal can only usefully throw rocks when there's a valuable target, so the extra brain might find other uses the rest of the time.
Hyperencephalization paused at three pounds. Either there's no more value to bigger brains, or the pelvis problem proved insuperable. That the pelvis problem is such a problem suggests that a bigger brain would be better, and selected for, otherwise. The high rate of death in childbirth would select against encephalization another way: to understand that one is likely to die that way ought to lead one to avoid reproducing. Maybe that's the real reason why women can't tell when they're fertile.
...suggests that a bigger brain would be better, ...
Let's stick wiht "bigger brain might/would be selected for? and leave out the "better" part!!!
But yea, probably the pelvis is a limiting factor.
Two questons from person not primate specialist. It seems to me that erect posture, requirning flattening of the pelvis, had to preced much brain enlargement, else a large enough birth canal would not have been present. Is this the case?
Many early homonids were small weak creatures with unimposing dentition. What was the set of cultural and technical adaptations which allowed them to survive? I think modern humans are able to throw harder and more accurately than any of the other present day primates. Because some humans make millions from throwing, I suspect the anatomy and physiology of a good thrower is well known. Has anyone looked at fossil homonids with this in mind?
It's the opposite with the birth canal. The birth canal of a typical primate is not a limiting factor on a pretty large baby. The erect posture makes the birth canal a limiting factor for hominids.
The early hominids were small but roubust and very muscular, like a modern chimp. They would have kicked butt. They did not have large canines, but otherwise their teeth were big and strong. But they could probably do amazing things with their hands, including possibly wielding an early version of the baseball bat or more impozing, of the base ball (made of rock!). (That is mainly, but not entirely, conjecture).
I'm not sure about the throwing difference. It has been suggested but so far I'm not buying it.
But to answer your question, yes! Barbara Isaac has looked very seriously into throwing and human evolution. Barbara is a very careful and thoughtful scholar. I mention this because others have looked into throwing who are not so careful.
I should blog Barbara's paper, really.
While researching a comment I decided not to post, I discovered the following that some readers might find interesting:
Nervous system organization (PPT)
Normal gross brain and microscopy
Just in case somebody else finds them useful.
Thanks for wanting to leave out the "better" regarding brain size. I've always felt uncomfortable with the suggestion that humans are the pinnacle and end (current or otherwise) result of evolution. Humans are having an increasingly devastating impact on all life on the planet. If we get off our thrones and look at it from what species are good for the continuation of life, humans would be near the end of the list.
Humans are more like a weed that has gone christian and is having a negative impact on everything including itself.
I wonder if human brains were larger whether the planet would be in better shape. We seem to have a problem with delayed gratification and too much talent at blocking out facts we don't want to face. Humans spend much less time overall doing great thinking as great rationalizing. I wonder if there is another animal candidate for developing a brain that doesn't result in messing up the planet.
Sounds interesting. I'd like to hear more about the tradeoffs between gracile and robust forms. I'm familiar with a couple of specific issues like MYH16 allowing for powerful chimp jaws but limiting cranial capacity. But in general, what are the disadvantages of a robust form? Reduction in fine dexterity? Higher caloric requirements?
Or is a third factor at work, such as robust forms being dependent upon hormonal levels that tend to inhibit social cooperation?
The hormonal thing may be a factor. But also, the robusticity is energetically costly, I would assume.
OH, but this has nothing to do with "gracile" vs. "robust" australopithecus. They were all robust.
I think it's a lot simpler than that.
At some point, on a living planet, one of the species may evolve a big brain.
At that point that species looks around and says "Hey! I'm the only one with a big brain!" and proceeds to develop technology and all sorts of craziness ensues.
We're that species.
"Better" is a reasonable shorthand for "positively selected in the given environment."
But yeah, for people who don't get that part, maybe it's confusing. Sort of like saying "the gecko foot has an excellent design for climbing vertical surfaces" makes most people think about function, while other people get stuck on the semantics and start thinking about supernatural engineers.
Putting aside the issue of human encephalization, there is a strong case to be made that large brains as a function of body size have a selection advantage.
1. The Cretaceous dinosaurs had larger brains as a function of body size then did their Jurassic antecedents.
2. Todays' mammals have larger brains as a function of body size then the mammals of 50 million years ago.
Yes! I'm not sure about these dinosaur brains, but there is a good argument that, for instance, carnivores outsmart herbivores and so the herbivores are selected to be a bit smarter, etc. etc. (or other similar interactive relationships develop) and you get a bit of an arms race going.
I don't know of a recent review of this idea, there is probably room for one.
I recall something about fruit-eating monkeys having larger brains than leaf-eating monkeys. Is it generally true that predators have larger brains than comparable size herbavores? Because I am largly carnivorious, I wonder if there is any difference in brain size among carnivore, vegitarian, and vegan humans.
There are dolphins with brain:body mass ratios very similar to humans. Great apes' brain:body ratios far exceed normal primate ratios. Whales have brain:body ratios that far exceed similar-sized giant sharks, so even if a small percentage the whale brain is used for cognition, the total brain power involved by weight would be more than we've got.
Looking at our "big brains" alone isn't something that makes us unique.
Jim: yes, you would have a big brain.
Brian: Mammals that give birth underwater have brains with lower neuron density than those that give birth on land, so the overall size of whale brains is exaggerated, in a sense.
Re dinosaurs
1. The best example of encephalization among cretaceous dinosaurs is the Trodons whose brains, as a function of body size were the largest of all dinosaurs so far discovered. Paleontologist Dale Russell has opined that, if they had not gone extinct at the KT boundary, they might have evolved into large brain birdlike creatures. In terms of morphological development, they were far ahead of the mammals, already being bipedal and having three fingers on their forelegs.
2. It is my information that the duck-billed dinosaurs had about the same brain sizes as the sauropods like the Apatosaurs but were some 5 times smaller in body weight.
AK's question about the problem of increased infant head size relative to the birth canal is crucial.
To pull numbers out of the air, let's say that a slightly larger brain increases maternal mortality by 1%. To persist, the mutation that increases brain size would have to confer a survival advantage of >1%.
Most of the proposed advantages of larger brains don't seem to fit the incrementalist model of natural selection: language would require a group of smarter hominids to be effective, tools found so far show no corresponding improvements, sexual selection would more likely favor females who preferred smaller-brained jocks, etc.
Moreover, the selective pressures against large brains would have increased dramatically in the later stages of the process: is there any non-domesticated mammal which has such difficulty with childbirth as homo sap?
My hypothesis is that some strange set of ecological circumstances propelled our ancestors through a particularly demanding evolutionary bottleneck over hundreds or thousands of generations, but my speculations on same haven't even reached the level of worthiness for an end-of-thread comment on a freewheeling blog.
tools found so far show no corresponding improvements,
The large increase in brain size that may have been enough to make birth dangerous is probably associated with the largest shift ever in technology, as well as (percentage wise) the largest expansion ever of hominds to new ranges and into new habitats.
Other domesticated mammals have a great deal of difficulty with childbirth (and that's a very good question, by the way) but I don't think we have the kind of data that makes this comparable for a number of reasons, including the fact that none of the mammalian domesticates are bipedal.
(and other reasons)
I wasn't going to address this, due to the time I would have to spend finding ref's, but since I can't post more than one or two links, I might as well summarize my own thoughts:
The way the regions of the cerebral cortex appear to work includes a complex transformation of a number of continuous scalar functions in n(1,2,3,4...) dimensions (the inputs) into an output (continuous) scalar function in no dimensions. Each pyramidal cell in a particular population in layers 2-3 provides a sampling of the output function at a particular location. There are multiple populations of pyramidal cells, each of which presumably performs a different calculation (of a different output).
Input axons from other cerebral regions, the thalamus, and sometimes other parts of the CNS similarly carry samplings of the input functions. The "White matter" of the cerebral hemispheres is made up of the myelinated axons carrying signals between regions. The "gray matter" is contains the cell bodies, unmyelinated axons, dendrites, synapses, extensive blood supply and other supporting systems. (See my earlier links for pictures.)
For example, in "The Astonishing Hypothesis", Crick describes the way in which inputs to the V1 region of the visual area, in which each axon carries a signal representing the likelihood that there is a spot of a particular type (size, color, etc.) at a particular point of the retina, are transformed into an output in which each axon carries a signal representing a bar of a particular type (size, orientation, color, etc.). This is a simple transformation to describe, and one of the few we seem to know about, but it is plausible that every region of the cerebral cortex performs similar transformations.
Consider the impact of increasing the number of inputs or outputs (pyramidal cells) for V1. Both the underlying white matter, and the cortex would increase in volume with increasing number of myelinated axons, cell bodies, and cross connections. But the "intelligence" of the calculation would not increase, only the precision.
Hypothetically, (or, perhaps speculatively,) a similar growth of overall brain size might support an increase in precision of some brain function(s) without any real increase in intelligence, while actual intelligence increases might be limited to improvements to the calculation algorithms each nerve cell performs.
A perfect example of the above, IMO, would be vocabulary. Although it couldn't be proven at this stage, it seems highly likely to me that the ability to remember large numbers of independent, arbitrary words is independent of any of the various abilities to work with them. This means that an increase in brain size might well be explained by a simple need for expanded vocabulary.
So I've been misinformed that artifacts dated to the proto-human brain-growth period were basically unchanged over those millennia, but tool-making change accelerated after that transition?
I doubt the item below is the only place I've read that claim, but it's what I can find at present:
Well, it's complicated. Here are some dates:"
A. Brains grow 20% to Homo Habilis/late australopith size at the same time that stone tools show up. The stone tools are not impressive. This is 2.5 mya
B. Brains triple in size, bodies double in mass, complexity of tools goes from breaking rocks to fashioning Acheulean hand axes, range of hominids goes from tropical and subtropical wooded savanna in Africa to inlcude semi arid regions, warmish temperate regions, and possibly more forested regions across all of Africa and most of southern Eurasia and occassionally father north, from Britian to East Asia (and Flores!???) at about 2.0 mya.
Brains go from 900 to 1100 CC's to 1100 to 1500 CC's, modest expansion of geographical range, significant expansion of range of habitats, one more order of magnitude of tool complexity (prepared platform technology) during the period of 500,000 years ago to 250,000 years ago. Earliest pre-Modern humans or actual modern humans, Neanderhtlas, etc. during and just subsequent to this period.
No change in human brain size, fully modern human geographical and habitat range, varying toolkits indicating fully modern range of technology with respect to the brain part (the technology gets to evolve on its own, of course, after this) from 250,000 to the origins of agriculture.
Possible drop in brain size ca 30,000
Probable drop in brain size in agricultural population starting 10,000
Predicted drop in brain size any day now unless the simulus package works.
Not quite enough data there to chart distinct lines for cranial capacity and tech levels, but it's sufficient to leave my premise about no correlation splattered across the rocks.
I think my point about the limits of birth canal width and brain size still stands, but will have to spend a while staring into space while ironing out wrinkles left by this incredible shrinking anomaly before I dare to raise the question again...
Perhaps I should erase all my notes from Dr. Shlain's intriguing but somewhat flaky book. (The blog he attempted on it, alas, was one of the most troll-ridden I've ever seen.)
@Greg:
You left out the artistic revolution of ca40-50KYA (Cro-Magnon), which was not associated with any increase in average brain size. Indeed, the "Possible drop in brain size ca 30,000" might have been associated with it.
That is the 30K thing, same thing. There is not an increase in brain size assocaited with that, and it didn't actualy happen ad advertized anyway. It is an outdated euro-centric concept.
Just found a good (if slightly old) reference regarding the functional organization of the human brain:
From sensation to cognition (PDF) by M.-Marsel Mesulam, Brain (1998), 121, 1013â1052
Another relevant paper:
A natural history of the human mind: tracing evolutionary
changes in brain and cognition by Chet C. Sherwood,
Francys Subiaul and Tadeusz W. Zawidzki; J. Anat. (2008) 212, pp426â454