When did we show up?

Genetically, humans are very similar to each other, the average difference being about 0.1%. Pan troglodytes (chimp) and Pan paniscus (bonobo) share with us (Homo) more than 98.8% of their DNA, more than they share with Gorilla, which shares 98.4% DNA with us. In terms of DNA, Pongo (Orangutan) differs from us by about 3%. So, the great African apes are very closely related and Homo and Pan especially so: there’s no living creature closer to us than Pan. All the great apes, Homo, Pan, Gorilla, and Pongo, differ from Rhesus monkeys by 7%.

Such figures, however, can be misleading because when features such as their gene locations or duplications are taken into account, the difference between us and Pan is about 6%. Still, because of such similarity, Pan, Homo, and Gorilla share a recent common ancestor. Molecular clock analysis tells us that ancestral Pan, which included the line leading to ancestral Homo diverged from ancestral Gorilla about 8 mya. The split in the ancestral Pan group between Homo and the chimp/bonobo line took place about 6 mya. For more, see this from the Smithsonian Institute.

The close similarity among Homo, Pan, and Gorilla (all African apes) provides further evidence that the CA was African.

There have been two great human migrations from Africa. One took place about 140,000ya. Studies on mDNA show that current Eurasians are not descendents of these groups of humans. The second migration took place about 60,000ya, and current Eurasians are descendents of these people.

Evolutionary parsimony dictates that an ancestor has the phenotypic and ultimately the genetic traits that are shared by all of its descendents (homology v. analogy); this gives rise to behavioral phylogenetics. For example, as chimps, bonobos, and humans are parochial, the ancestral Pan was

1. Parochial

2. By similar psychological mechanisms

3. Brought about by the same genetic endowment.

So, we can infer the ancestral Pan

· Had some sense of the self, an understanding of the intentions of others, and the ability to engage in perspective thinking.

· Had a matricentric family

· Could be aggressive towards members of the same group

· Lived in hierarchically structured groups with alpha male, political coalitions, and a culture

· Did not like being dominated and occasionally would form coalitions to castigate or even expel alpha males that were too bossy, thus exercising some degree of (probably non-moral) social control.

· Was capable of following complex social rules of behavior that were culturally transmitted

· Was mainly vegetarian but occasionally hunted for small animals.

Some of the above traits can certainly be found in the Common Ancestor of about 8 mya.

It’s unlikely the ancestral Pan was also capable of internalizing norms, as non human apes can use language containing “good,” “bad,” “sorry,” but there is no indication that they attach any moral sense to them. Anecdotal guilt-tripping of non-human apes can be naturally explained in terms of the ape accommodating the feeling of a close bonded human.

The earliest certain human ancestor is Homo erectus, which appeared about 2 mya, with a brain about half the size of ours; archaic forms of Homo sapiens were hunting about 400,000 bp and at about 250,000 bp they were systematically group hunting large ungulates.

Homo sapiens (us):

· Anatomically modern humans appear about 200,000 years ago (we know this on the basis of bones dating and molecular clock analysis)

· Culturally modern humans (complex artifacts, adornments, art): about 50,000 years ago. However, the date may be 75,000 years or earlier.

Human society in the late Pleistocene and early Holocene

Archeological evidence (little) and inferences from ethnographic hunter gatherers indicates that for 90% (or more) of our existence on Earth as modern humans we lived as

· Hunter-gatherers

· In demes typically constituted by 50 or so that lasted only for a few generations

· With high rates of migration from one deme to another because of exogamy or coinsurance, as evidenced by (typical but not always present) small inter-deme genetic distance among ethnographic hunter gatherers.

· As civic minded members of our deme engaged in strong reciprocity

· Often at war with other demes because of scarcity of resources brought about by climactic variations (mean temperatures changing of 8 C in two or three hundred years).

· Often subject to cataclysmic events so that our growth rate was small or nil even if the potential growth of ethnographic hunter gatherer societies is about 2% per annum

Parochial altruists and war

Humans are prone to the creation of group boundaries and are parochial in the sense of engaging in in-group favoritism in the choice of friends, exchange partners, and allocation of valuable resources. This does not mean that parochialism must result in overt hostility or actual war.

Like altruism, parochialism is costly because it deprives one of gain opportunities by reducing the pool of exchange partners. Consequently, parochialism alone is unlikely to be evolutionarily successful. However, parochial altruism can be successful as long as inter-group relations are sufficiently hostile.

B&G construct a model in which parochialism and altruism coevolve and endogenously produce inter-group hostilities, which is turn synergistically makes parochial altruism successful.

B&G’s Model

Agent based, with parameters such as mutation and migration calibrated to fit the environment of late Pleistocene/early Holocene humans.

Parochialism and altruism are taken to be genetically based, producing four types:

· Parochial altruists (these are fighters)

· Non-parochial altruists

· Parochial non-altruists

· Non-parochial non-altruists

Altruists pay a cost c producing a public good b equally shared among n adults in the group (average group size is 26). We assume b > c > b/n. In the simulation, c= .01, b=.02. n is about 30; g= .001 is the benefit of non-hostile group interaction

In the absence of hostile group interaction, the payoffs are

· PA: b(fraction of n group members who are A) – c

· P~A: b(fraction of n group members who are A)

· ~PA: b(fraction of n group members who are A) – c + g( number of members of the other group x fraction of ~P in that group)

· ~P~A: b(fraction of n group members who are A) + g( number of members of the other group x fraction of ~P in that group)

When P constitute a sufficiently large fraction of a group, the group has hostile relations with other groups. When the imbalance between the PA of two groups is large enough, they go to war, and the group with a favorable balance of PA wins with greater probability. A fraction of fighters set at .14 dies. The fraction of the civilian members of the losing group eliminated is proportional to the imbalance; the members eliminated are replaced with copies of randomly chosen members of the winning group.

Individuals are haploid but trait transmission with mutation (m = .005) is diploid; reproduction is proportional to accumulated payoff; P, ~P, A, ~A are transmitted separately and each trait has 50% probability of transmission. Migration rate is .3 per generation.

Results

Over many repetitions, the stationary distribution is ergodic and bimodal (it looks like a U) with a small saddle point, with either ~P~A or PA between 40% and 50% of the population. So, any run very likely ends up with one of the two alternatives, one of which endogenously produces high levels of inter-group wars.

In fact, when PA are about 50% on average a group

· is at war every 7 generations, which means that the probability of being at war in each generation is .14

· if it wins it suffers about 10% fatalities, all PA

· if it loses, it suffers about 40% fatalities

Hence, in the neighborhood of PA= 50% about 3.5% of the population dies in war per generation. Such levels of conflict and death are considerably below the estimates from archeology and ethnography, which indicates an exogenous war intensifier, probably late Pleistocene climate instability.

When PA is high, hostility among groups is intense and war likely. War is most common when PA is between 30% and 80%: below 30% hostile group interactions are rare and PA are disadvantaged; above 80% the imbalance in terms of PA among groups is too small to produce wars.

So, P and A coevolve, endogenously producing war and being themselves selected when war occurs. Our warlike tendencies are caused by, and cause, parochial altruism. Moreover, the high climate variability that characterized the late Pleistocene favored the rise of PA in the population.

This does not entail that P and A are connected by necessity, although the tie is certainly disturbing.