For two decades or more, biologists have had a problem with astronomical and geological accounts of the early sun's evolution. The problem is that our models show that the sun was probably too faint to maintain liquid water on the Earth's surface until sometime in the Paleoarchaean, and solid ice makes a poor chemical reactor for early evolution. In 2007 Daupha et al showed evidence of mineral deposition 3.8 Ga under conditions of high-pressure carbon dioxide, which of course means a probable greenhouse effect at that time on Earth.
Because we've always assumed early Earth was a hot Earth, the conventional wisdom of the evolution of life on Earth has held that the last universal common ancestor (LUCA) of all living things was a thermophile, much like the biochemically weird things living in deep sea vents or those prismatic mats at Yellowstone, which is why an early cold Earth doesn't seem to fit. But a snowball or at least near-snowball ancient Earth may not be a problem after all. A recent publication by Boussau et al shows that LUCA was a mesophile, best adapated to life at temperatures below 10 degrees C.
This is also consistent with the idea of an RNA world, since RNA is unstable at low and high pH, and even less stable at high temperatures than DNA. Interestingly, the LUCA of eubacteria and archaebacteria does look like a thermophile, consistent with temperature estimates of later periods in Earth's history when the ocean was thought to be on average 80 degrees C (about 3.5 Ga). The resolution of ancestors at different time depths is permitted by the use of two methods, one based on GC content of the genome and one based on amino acid substitution.
This is biological and geological convergence at its finest; Lyell and Darwin (and Woehler and Mendel) would be proud. We now have a consistent story for the evolution of the sun, the atmosphere of early Earth and its effect on surface temperature, the temperature adaptation of the earliest cells and the switch from RNA to DNA. The next step is to explore the period 3.8 to 3.5 Ga, to look for evidence of skyrocketing ocean temperatures; not inconceivable given a thick CO2 atmosphere and a sun that was still booting up. If our picture of Earth's surface temperature in that period is accurate, we can also start to ask whether rising temperatures were the selection pressure switching the early biosphere from RNA to DNA.
Tuesday, December 23, 2008
Sunday, December 21, 2008
Carnivory and Prestige in Primates: Hunting as a Form of Forced Capital
The first thinker who clearly stated the importance of capital to economic growth was the eighteenth century French economist Turgot. Turgot was a physiocrat, from a school of early economic thought that Adam Smith admired for its rigor and hard-headedness but criticized for its conclusions - chiefly among them, that the land and its products solely determined value, and labor was negligible. The physiocratic economists may seem narrow-minded simpletons by modern economic lights, but they brought a materialist Enlightenment thinking to a social science that was in dire need of it. When Turgot phrased the theory of capital this way, in perhaps its earliest explicit conception, its historical development becomes clearer. Although Smith and later thinkers have formulated the concept more elegantly, Turgot's conception was, in the abstract, the same one we have today.
If it seems to you there is no obvious link from the concept of capital to the anthropological problems of hunter prestige and human meat-eating, you're right. It wasn't until I read Turgot's explanation of capital in food terms that I made the connection:
As soon as men were found whose ownership of land assured them of an annual revenue more than sufficient to meet all their needs, there were bound to be men who, either because they were anxious about the future or merely because they were prudent, put into reserve a portion of what they gathered in each year….When the produce which they gathered in was difficult to keep, they must have sought to obtain for themselves in exchange objects of a more durable nature whose value would not be lost with time...
Section XLIX, Of the reserve of annual produce, accumulated to form capitals, Reflections on the Formation and Distribution of Wealth, 1766
The physiocratic conception of capital boils down to: land produces surplus harvest; surplus harvest can be stored or (even better since it's perishable) traded. Without this surplus to "get ahead" there can never be any enterprise which does not pay for itself in the short term, even if said enterprise creates wealth in the long term. Therefore the accumulation of surplus wealth by surplus harvest encourages further wealth creation and is desirable.
We can forgive Turgot his eighteenth-century view that the value-adding power of labor was negligible; indeed, our ability to add value through work has increased dramatically since then, and Smith's forward-looking focus on the labor component has become correspondingly more crucial. Another way of saying this is that the further back in time you go, the closer to the truth physiocracy actually was. What's more interesting here is the idea of harvest surplus providing an opportunity for eighteenth century farmers to accumulate wealth. It's not difficult to extend Turgot's formulation to even earlier farmers in the late neolithic Middle East. A farmer might produce an unexpected bumper crop and, ironically, panic at his good fortune - here, suddenly, is extra food that can either get eaten by mice, or given away. If the farmer gives it away, of course he would like to do so to his own benefit. An astute farmer will then quickly send a runner to the next valley over with an offer to trade for pottery - or arrows - or for the people there to come and harvest it for themselves, on condition of future favors. Because the product is perishable, the unexpected surplus had become forced capital - and drove trade much like a stone age stimulus check.
Push this concept one step further back, to hunter-gatherers, and you make the connection between capital and the seemingly disparate problem of human omnivorousness. There are two problems with human carnivory. The first problem is that we eat meat at all. Humans are in an order of mammals (primates) that is almost totally herbivorous, and yet last night I had steak. What's going on? In fact we are an outgroup among primate species in our high reliance on meat for calories. Today Americans get about 10% of our calories from meat; for non-agricultural hunter-gatherers it's usually around 20%,(1) which is probably closer to our ancestral diet for the last few hundred thousand years. In fact some hunter-gatherers like the Ache in Paraguay are estimated to have received in pre-contact times more than 55% of their calories from meat.(2) In contrast, our closest living relatives (chimpanzees) are also the next-most-carnivorous primate, although the proportion of meat calories in their diet is only 1.4% to 13% (the two extreme estimates).(3,4) At least by two million years ago, some group of hominids started eating meat as a regular part of their diet. Why then? An explanation involving macronutrient requirements is unsatisfying, because it begs the question of why the requirement would suddenly have appeared. The problem persists because hunters can often be shown to be acting sub-optimally - that is, they could be bringing home more calories per time expended if they were foraging for plant material.(5)
The second problem is understanding why hunters are accorded high social status. Eating meat actually has many disadvantages - taking down big game is hazardous to your health, success is inconsistent, calories per unit time is lower, and as a food meat doesn't keep as well as fruit or grains. Yet despite this, high status for successful hunters is universal among cultures that hunt, as often demonstrated by access to fecund females. Perhaps the most striking example is the Lakota buffalo-calling tradition encountered most famously by Lewis and Clarke, in which a young, unsuccessful hunter tries to obtain spirit power from older, successful hunter by transfer through the younger hunter's wife. The crucial detail in buffalo-calling is that the transfer occurs through sexual intercourse. This is a sweet deal for the older hunter, who spreads his genes with little effort by getting the younger hunter to raise his children. Indeed, if in other activities, like food-sharing, hunters can be shown to be increasing their inclusive fitness concomitant with the meat calories they bring in to the family, the mystery is solved.
It turns out that inclusive fitness doesn't explain it either. Gurven et al looked at food-sharing practices among the Hiwi, savannah-dwellers in Venezuela and Colombia.(6) Thinking back to Turgot for a moment, they Hiwi cannot have agricultural surpluses because they do not have agriculture; they're hunter-gatherers. But the physiocratic conception of capital still applies. What the anthropologists found is that hunters share out meat from large game (like peccary, a South American boar) in larger pieces, and to more distantly-related people, than they do other calorie sources.
This seems to fly in the face of ideas of inclusive fitness, but it makes sense if you consider why hunting big game might be advantageous. A Hiwi hunter who brings in a big peccary (or for that matter a successful Lakota hunter who brings in a buffalo) has the same problem that our neolithic farmer had with his surprise bumper crop. The Hiwi has 100 kilograms of meat, and in the hot savannah it will surely be bad in a few days; even if his family takes their fill, most will go to waste. A hunter that shares just because he's nice will receive no benefit in return, and you won't expect to see any advantage to his genes in the next generation. On the other hand, a hunter clever enough to share with unrelated individuals in exchange for favors - that is, who trades - will have a strong advantage indeed, because he can turn extra meat into value. Big game among the Hiwi today, and among all of us in the paleolithic, meant the same thing that agriculture would mean in the neolithic - forced capital accumulation through trading a surplus of perishable goods.
The forced-capital theory of big-game hunting may offer a clue not only to the prestige problem, but to the problem of why we starting eating meat when we started eating meat. The macronutrient supplement explanation fails to account for why there is only one genus (our own) that became strongly omnivorous, when there were clearly other primates under nutritional stress; it's also worth asking why, if we were already eating meat at least two million years ago, we didn't start seeing changing niche boundaries until five hundred thousand years ago.(7) But there are other traits where humans comparatively stand out among primates, chiefly among them social intelligence. Crucially, the forced capital phenomenon couldn't benefit good hunters until humans became smart enough to keep track of who returned the favor - that is, until hominids could maintain credit reports on all the tribe members who'd delivered or reneged on their promises.
One way to test this idea is to look at the relationship between primate species intelligence and percentage of meat calories in the species diet; the smarter a primate, the better chance it has to benefit as a hunter by keeping track of its sharing, and therefore more opportunity to convert meat calories into capital (goods and favors later). There is an established relationship between primate group size and brain size (the bigger your brain, the smarter you are and the more social relationships you can keep track of),(8) so we can think of both as proxy measures for intelligence.
For baboon human, and chimp data see (9)
For gorilla and orangutan data see (10)
The conclusion isn't earth-shattering; the link between meat consumption and intelligence isn't news. Craig Stanford explored it thoroughly in The Hunting Apes, although he did show an interesting relation that is worth emphasizing. In both Ache humans and chimps, there was a relationship between meat obtained per-hunter and hunting party size. Very small or very large hunting parties get more meat per hunter, with a "dead zone" in the middle. The differences were less exaggerated for chimps than for humans.(11)
It's probably not useful to think of social intelligence, hunting, and dependence on meat as a chicken-and-egg problem. First, it is underappreciated that commitment to carnivory is requires less in terms of digestive hardware than commitment to herbivory. That is to say, a deer can digest meat more easily than a tiger can digest grass. This is a basic fact of chemistry: cellulose, the principal component by mass of plants, is much more stable than protein. Consequently obligate herbivores require massive large intestines (the size of gorillas has been interpreted by some as largely driven by their increasing restriction to a low-quality diet) whereas obligate predators typically have a reduced cecum, or no cecum at all. It's best to envision the process whereby humans gradually take advantage of hunting opportunities as a feedback loop between brain size, social group size, and availability of protein calories from game in new environments. It's not even necessary to think of humans as bringing down big game. There marks from cutting tools have been found on paleolithic big game bones in Africa, but Stanford and others have pointed out that the cut marks are on top of damage from large carnivoress. That is, these humans were scavengers. By 20 kA ago, we have our first evidence of organized large game hunts. Other adaptations allowed the final expansion of brainpower that allowed the coordination of large hunts that make a more important difference to meat-yield in humans than in chimps
It bears repeating that not only the ability to organize hunts, but also the ability to effectively control and share meat, would have favored increased hunting activity. Chimps occasionally pursue gazelles and some Neandertal injuries are similar to those of rodeo-riders who fall or get kicked by large animals, suggesting they were attacking the animals at close range. It was not just the ability to hunt, but the ability to share effectivvely that most impacted hunters' fitness. A direction for further research is to try to show how the appearance of coordinated large game hunts matches with the spread of new artifacts and behavior-affecting genes, like FoxP2 and specialized-function cutting tools, which both appeared around 40,000 kA.
What would be most supportive of the theory would be a large-neocortex, high-meat consuming primate that doesn't also happen to be our closest relative, as in the case of chimps. However it's still interesting that besides humans, chimps are the only primates that share meat without fighting(12). Chimps also share meat with nonkin; an inclusive fitness explanation only seems mandatory if you don't consider meat-sharing with nonkin to have any reciprocation benefits. Still, it's anything but certain that this is done in expectation of future reciprocity.(13)
Several counterarguments arise in addition to the nutritional stress/macronutrient adaptation points made before. First, it is possible that as an herbivorous species becomes more intelligent, it has the opportunity to become more generalist, and therefore it is more likely to change its diet to include meat. This explanation does not, however, account for hunter prestige, and it does not account for the general human distaste for extremely fresh meat. It has also been argued that hominids began eating meat from kills left by large African carnivores during periods of caloric stress (as baboons do now), but this begs the question of why a hunter who brings back meat that he himself had recently killed meat would be accorded special status.
It is possible that these other explanation provided the initial impetus for early hominids to supplement the diet with meat, and that the forced-capital model was a fortuitous phenomenon which accelerated the evolution of hominid carnivory. It is intriguing to consider that meat consumption by humans (and prestige of hunters) developed because large game represented forced capital accumulation in a primate that had become intelligent enough to remember which nonkin reciprocated its meat trades. Turgot's physiocratic position that the land produces all wealth is literally true for primates and human hunter-gatherers, and his conception of capital is most obviously the same one that a neolithic farmer or a meat-trading chimpanzee is using. That this behavior has appeared only in primates that live in large groups strongly suggests that it relies on the development of a greater social intelligence.
REFERENCES
(1) E.N. Anderson. Everyone Eats, 2005.
(2) K. Hill. Encyclopedia of World Cultures, Vol. 4, 1994.
(3) J. Goodall. The Chimpanzees of Gombe, 1986.
(4) K. Milton. A hypothesis to explain the role of meat-eating in human evolution. Evol. Anthrop. 8: 11–21 (1999).
(5) K. Hill. Macronutrient modifications of optimal foraging theory: An approach using indifference curves applied to some modern foragers, Human ecology, Vol. 16 Number 2 June 1988.
(6) M. Gurven et al. Food Transfers Among Hiwi Foragers of Venezuela: Tests of Reciprocity. Human Ecology, Vol. 28, No. 2, 2000.
(7) Mary C. Stiner. Carnivory, Coevolution, and the Geographic Spread of the Genus Homo. Journal of Archaeological Research, 1573-7756, Volume 10, Number 1 / March, 2002.
(8) Byrne and Whiten. Machiavellian Intelligence: Social Expertise and the Evolution of Intelligence in Monkeys, Apes and Humans, 1988.
(9) H. Kudo and R.I.M. Dunbar. Neocortex Size and Social Network Size in Primates, Animal Behavior, 2001, 61.
(10) Craig Stanford, The Hunting Apes: Meat-Eating and the Origins of Human Behavior p. 147. 1999.
(11) (Preprint) Dunbar, R. I. M. Coevolution of neocortical size, group size and language in humans, Behavioral and Brain Sciences 16 (4): 681-735, 1993.
(12) RS Harding. Ranging Patterns of a Troop of Baboons (Papio anubis) in Kenya.
Folia Primatologica 1976;25:143-185.
(13) I.C. Gilby. Meat sharing among the Gombe chimpanzees: harassment and reciprocal exchange. Animal Behaviour 71: 953-963, 2006.
If it seems to you there is no obvious link from the concept of capital to the anthropological problems of hunter prestige and human meat-eating, you're right. It wasn't until I read Turgot's explanation of capital in food terms that I made the connection:
As soon as men were found whose ownership of land assured them of an annual revenue more than sufficient to meet all their needs, there were bound to be men who, either because they were anxious about the future or merely because they were prudent, put into reserve a portion of what they gathered in each year….When the produce which they gathered in was difficult to keep, they must have sought to obtain for themselves in exchange objects of a more durable nature whose value would not be lost with time...
Section XLIX, Of the reserve of annual produce, accumulated to form capitals, Reflections on the Formation and Distribution of Wealth, 1766
The physiocratic conception of capital boils down to: land produces surplus harvest; surplus harvest can be stored or (even better since it's perishable) traded. Without this surplus to "get ahead" there can never be any enterprise which does not pay for itself in the short term, even if said enterprise creates wealth in the long term. Therefore the accumulation of surplus wealth by surplus harvest encourages further wealth creation and is desirable.
We can forgive Turgot his eighteenth-century view that the value-adding power of labor was negligible; indeed, our ability to add value through work has increased dramatically since then, and Smith's forward-looking focus on the labor component has become correspondingly more crucial. Another way of saying this is that the further back in time you go, the closer to the truth physiocracy actually was. What's more interesting here is the idea of harvest surplus providing an opportunity for eighteenth century farmers to accumulate wealth. It's not difficult to extend Turgot's formulation to even earlier farmers in the late neolithic Middle East. A farmer might produce an unexpected bumper crop and, ironically, panic at his good fortune - here, suddenly, is extra food that can either get eaten by mice, or given away. If the farmer gives it away, of course he would like to do so to his own benefit. An astute farmer will then quickly send a runner to the next valley over with an offer to trade for pottery - or arrows - or for the people there to come and harvest it for themselves, on condition of future favors. Because the product is perishable, the unexpected surplus had become forced capital - and drove trade much like a stone age stimulus check.
Push this concept one step further back, to hunter-gatherers, and you make the connection between capital and the seemingly disparate problem of human omnivorousness. There are two problems with human carnivory. The first problem is that we eat meat at all. Humans are in an order of mammals (primates) that is almost totally herbivorous, and yet last night I had steak. What's going on? In fact we are an outgroup among primate species in our high reliance on meat for calories. Today Americans get about 10% of our calories from meat; for non-agricultural hunter-gatherers it's usually around 20%,(1) which is probably closer to our ancestral diet for the last few hundred thousand years. In fact some hunter-gatherers like the Ache in Paraguay are estimated to have received in pre-contact times more than 55% of their calories from meat.(2) In contrast, our closest living relatives (chimpanzees) are also the next-most-carnivorous primate, although the proportion of meat calories in their diet is only 1.4% to 13% (the two extreme estimates).(3,4) At least by two million years ago, some group of hominids started eating meat as a regular part of their diet. Why then? An explanation involving macronutrient requirements is unsatisfying, because it begs the question of why the requirement would suddenly have appeared. The problem persists because hunters can often be shown to be acting sub-optimally - that is, they could be bringing home more calories per time expended if they were foraging for plant material.(5)
The second problem is understanding why hunters are accorded high social status. Eating meat actually has many disadvantages - taking down big game is hazardous to your health, success is inconsistent, calories per unit time is lower, and as a food meat doesn't keep as well as fruit or grains. Yet despite this, high status for successful hunters is universal among cultures that hunt, as often demonstrated by access to fecund females. Perhaps the most striking example is the Lakota buffalo-calling tradition encountered most famously by Lewis and Clarke, in which a young, unsuccessful hunter tries to obtain spirit power from older, successful hunter by transfer through the younger hunter's wife. The crucial detail in buffalo-calling is that the transfer occurs through sexual intercourse. This is a sweet deal for the older hunter, who spreads his genes with little effort by getting the younger hunter to raise his children. Indeed, if in other activities, like food-sharing, hunters can be shown to be increasing their inclusive fitness concomitant with the meat calories they bring in to the family, the mystery is solved.
It turns out that inclusive fitness doesn't explain it either. Gurven et al looked at food-sharing practices among the Hiwi, savannah-dwellers in Venezuela and Colombia.(6) Thinking back to Turgot for a moment, they Hiwi cannot have agricultural surpluses because they do not have agriculture; they're hunter-gatherers. But the physiocratic conception of capital still applies. What the anthropologists found is that hunters share out meat from large game (like peccary, a South American boar) in larger pieces, and to more distantly-related people, than they do other calorie sources.
This seems to fly in the face of ideas of inclusive fitness, but it makes sense if you consider why hunting big game might be advantageous. A Hiwi hunter who brings in a big peccary (or for that matter a successful Lakota hunter who brings in a buffalo) has the same problem that our neolithic farmer had with his surprise bumper crop. The Hiwi has 100 kilograms of meat, and in the hot savannah it will surely be bad in a few days; even if his family takes their fill, most will go to waste. A hunter that shares just because he's nice will receive no benefit in return, and you won't expect to see any advantage to his genes in the next generation. On the other hand, a hunter clever enough to share with unrelated individuals in exchange for favors - that is, who trades - will have a strong advantage indeed, because he can turn extra meat into value. Big game among the Hiwi today, and among all of us in the paleolithic, meant the same thing that agriculture would mean in the neolithic - forced capital accumulation through trading a surplus of perishable goods.
The forced-capital theory of big-game hunting may offer a clue not only to the prestige problem, but to the problem of why we starting eating meat when we started eating meat. The macronutrient supplement explanation fails to account for why there is only one genus (our own) that became strongly omnivorous, when there were clearly other primates under nutritional stress; it's also worth asking why, if we were already eating meat at least two million years ago, we didn't start seeing changing niche boundaries until five hundred thousand years ago.(7) But there are other traits where humans comparatively stand out among primates, chiefly among them social intelligence. Crucially, the forced capital phenomenon couldn't benefit good hunters until humans became smart enough to keep track of who returned the favor - that is, until hominids could maintain credit reports on all the tribe members who'd delivered or reneged on their promises.
One way to test this idea is to look at the relationship between primate species intelligence and percentage of meat calories in the species diet; the smarter a primate, the better chance it has to benefit as a hunter by keeping track of its sharing, and therefore more opportunity to convert meat calories into capital (goods and favors later). There is an established relationship between primate group size and brain size (the bigger your brain, the smarter you are and the more social relationships you can keep track of),(8) so we can think of both as proxy measures for intelligence.
For baboon human, and chimp data see (9)
For gorilla and orangutan data see (10)
The conclusion isn't earth-shattering; the link between meat consumption and intelligence isn't news. Craig Stanford explored it thoroughly in The Hunting Apes, although he did show an interesting relation that is worth emphasizing. In both Ache humans and chimps, there was a relationship between meat obtained per-hunter and hunting party size. Very small or very large hunting parties get more meat per hunter, with a "dead zone" in the middle. The differences were less exaggerated for chimps than for humans.(11)
It's probably not useful to think of social intelligence, hunting, and dependence on meat as a chicken-and-egg problem. First, it is underappreciated that commitment to carnivory is requires less in terms of digestive hardware than commitment to herbivory. That is to say, a deer can digest meat more easily than a tiger can digest grass. This is a basic fact of chemistry: cellulose, the principal component by mass of plants, is much more stable than protein. Consequently obligate herbivores require massive large intestines (the size of gorillas has been interpreted by some as largely driven by their increasing restriction to a low-quality diet) whereas obligate predators typically have a reduced cecum, or no cecum at all. It's best to envision the process whereby humans gradually take advantage of hunting opportunities as a feedback loop between brain size, social group size, and availability of protein calories from game in new environments. It's not even necessary to think of humans as bringing down big game. There marks from cutting tools have been found on paleolithic big game bones in Africa, but Stanford and others have pointed out that the cut marks are on top of damage from large carnivoress. That is, these humans were scavengers. By 20 kA ago, we have our first evidence of organized large game hunts. Other adaptations allowed the final expansion of brainpower that allowed the coordination of large hunts that make a more important difference to meat-yield in humans than in chimps
It bears repeating that not only the ability to organize hunts, but also the ability to effectively control and share meat, would have favored increased hunting activity. Chimps occasionally pursue gazelles and some Neandertal injuries are similar to those of rodeo-riders who fall or get kicked by large animals, suggesting they were attacking the animals at close range. It was not just the ability to hunt, but the ability to share effectivvely that most impacted hunters' fitness. A direction for further research is to try to show how the appearance of coordinated large game hunts matches with the spread of new artifacts and behavior-affecting genes, like FoxP2 and specialized-function cutting tools, which both appeared around 40,000 kA.
What would be most supportive of the theory would be a large-neocortex, high-meat consuming primate that doesn't also happen to be our closest relative, as in the case of chimps. However it's still interesting that besides humans, chimps are the only primates that share meat without fighting(12). Chimps also share meat with nonkin; an inclusive fitness explanation only seems mandatory if you don't consider meat-sharing with nonkin to have any reciprocation benefits. Still, it's anything but certain that this is done in expectation of future reciprocity.(13)
Several counterarguments arise in addition to the nutritional stress/macronutrient adaptation points made before. First, it is possible that as an herbivorous species becomes more intelligent, it has the opportunity to become more generalist, and therefore it is more likely to change its diet to include meat. This explanation does not, however, account for hunter prestige, and it does not account for the general human distaste for extremely fresh meat. It has also been argued that hominids began eating meat from kills left by large African carnivores during periods of caloric stress (as baboons do now), but this begs the question of why a hunter who brings back meat that he himself had recently killed meat would be accorded special status.
It is possible that these other explanation provided the initial impetus for early hominids to supplement the diet with meat, and that the forced-capital model was a fortuitous phenomenon which accelerated the evolution of hominid carnivory. It is intriguing to consider that meat consumption by humans (and prestige of hunters) developed because large game represented forced capital accumulation in a primate that had become intelligent enough to remember which nonkin reciprocated its meat trades. Turgot's physiocratic position that the land produces all wealth is literally true for primates and human hunter-gatherers, and his conception of capital is most obviously the same one that a neolithic farmer or a meat-trading chimpanzee is using. That this behavior has appeared only in primates that live in large groups strongly suggests that it relies on the development of a greater social intelligence.
REFERENCES
(1) E.N. Anderson. Everyone Eats, 2005.
(2) K. Hill. Encyclopedia of World Cultures, Vol. 4, 1994.
(3) J. Goodall. The Chimpanzees of Gombe, 1986.
(4) K. Milton. A hypothesis to explain the role of meat-eating in human evolution. Evol. Anthrop. 8: 11–21 (1999).
(5) K. Hill. Macronutrient modifications of optimal foraging theory: An approach using indifference curves applied to some modern foragers, Human ecology, Vol. 16 Number 2 June 1988.
(6) M. Gurven et al. Food Transfers Among Hiwi Foragers of Venezuela: Tests of Reciprocity. Human Ecology, Vol. 28, No. 2, 2000.
(7) Mary C. Stiner. Carnivory, Coevolution, and the Geographic Spread of the Genus Homo. Journal of Archaeological Research, 1573-7756, Volume 10, Number 1 / March, 2002.
(8) Byrne and Whiten. Machiavellian Intelligence: Social Expertise and the Evolution of Intelligence in Monkeys, Apes and Humans, 1988.
(9) H. Kudo and R.I.M. Dunbar. Neocortex Size and Social Network Size in Primates, Animal Behavior, 2001, 61.
(10) Craig Stanford, The Hunting Apes: Meat-Eating and the Origins of Human Behavior p. 147. 1999.
(11) (Preprint) Dunbar, R. I. M. Coevolution of neocortical size, group size and language in humans, Behavioral and Brain Sciences 16 (4): 681-735, 1993.
(12) RS Harding. Ranging Patterns of a Troop of Baboons (Papio anubis) in Kenya.
Folia Primatologica 1976;25:143-185.
(13) I.C. Gilby. Meat sharing among the Gombe chimpanzees: harassment and reciprocal exchange. Animal Behaviour 71: 953-963, 2006.
Subscribe to:
Posts (Atom)
