Genetic perspectives on prehistoric social practices

Brigitte PakendorfMPI for Evolutionary Anthropology, Leipzig, Germany

Social practices and genetics?

• Social practices can have an effect on the number of offspring a person has can be detected with genetic methods

• Holds especially true for social practices that affect one of the sexes more than the other: polygyny, residency patterns, ‘upward’ social mobility

Benefits of mtDNA and Y chromosome

• exclusively maternal and paternal inheritance, respectively

• no recombination

The benefits of studying mtDNA and Y-chromosomal markers:

Y(non-sex) chromosome

mtDNA

Y-chromosome

Benefits of mtDNA and Y chromosome

→ mutations accumulate with time alone→ possibility of defining related lineages (=

haplogroups), i.e. shared mutations indicate shared ancestry (if mutations are rare!!)

→ complementary studies of population history (female vs male) are possible

Types of mutations

• SNPs = Single Nucleotide Polymorphisms – individual base changes (e.g. T C):slow mutation rate

• STRs = Short Tandem Repeats – change in number of repeat sequence of bases (e.g. [AGCT]16 [AGCT]17):very high mutation rate

Socially conditioned prehistoric events

1. Residence patterns

2. Sex-biased migrations

3. Polygamy

4. ‘Upward’ social mobility

1) Residence patterns

• Matrilocality: the groom settles with the wife’s family after marriage mixing of Y-chromosomes, mtDNA’s stay put

• Patrilocality: the bride settles with the husband’s family after marriage mixing of mtDNA’s, Y-chromosomes stay put

• Prediction: in patrilocal groups, mtDNA diversity should be higher than Y-chromosomal diversity, and vice versa for matrilocal groups

Oota et al. (2001) Nature Genetics 29: 20 - 21

Genetic diversity in Thailand hill tribes

Melanesian mtDNAAsian mtDNAOther mtDNA

2) Sex-biased migrations: Pacific

Kayser et al. (2006) Mol Biol Evol. 23: 2234-44.Map courtesy of the University of Texas Libraries, The University of Texas at Austin.

Melanesian Y-DNAAsian Y-DNAOther Y-DNA

2) Sex-biased migrations: Pacific

Kayser et al. (2006) Mol Biol Evol. 23: 2234-44.Map courtesy of the University of Texas Libraries, The University of Texas at Austin.

Polynesian mtDNA: 94% Asian origin

2) Sex-biased migrations: Pacific

Polynesian Y: 66% Melanesian origin

Potentially due to matrilocality of Austronesian- speakersMelanesian men incorporated into Austronesian- speaking society prior to further migration to Polynesia

Kayser et al. (2006) Mol Biol Evol. 23: 2234-44.

2) Sex-biased migrations: male conquerors

• Closely-related Y-chromosomal lineage identified in 16 Central Asian populations

Zerjal et al. (2003): Am J Hum Gen. 72: 717–721

2) Sex-biased migrations: male conquerors

Zerjal et al. (2003): Am J Hum Gen. 72: 717–721

2) Sex-biased migrations: male conquerors

• Closely-related Y-chromosomal lineage identified in 16 Central Asian populations

• Dated to ~ 700-1300 years BP• Most likely origin in Mongolia (highest diversity)

Zerjal et al. (2003): Am J Hum Gen. 72: 717–721

2) Sex-biased migrations: male conquerors

Distribution of Mongolian Y-chromosomal lineage; shaded area = extent of Mongol Empire at time of Chinggis Khan’s death

Zerjal et al. (2003): Am J Hum Gen. 72: 717–721

2) Sex-biased migrations: male conquerors

• Y-lineage with one male ancestor ~ 1000 years ago

• widespread in Central Asia spread with Mongol Empire

• ruling clans = Chinggis Khan’s sons and grandsons ‘Chinggis Khan’s Y-chromosome’

Zerjal et al. (2003): Am J Hum Gen. 72: 717–721

3) Polygamy (polygyny)

• Polygyny: few men have many wives, and many men may have no wife at all

• Prediction: (severely) reduced Y-chromosomal diversity

Kayser et al. (2003): Am J Hum Genet 72:281-302

Y-chromosomal SNP frequencies in New Guinea

Y-chromosomal and mtDNA diversity, West New Guinea

Group (N Y/ N mtDNA)

Y-SNP diversity

Y-STR diversity

HVR1 diversity

Dani (12/21) .167 ± .134 .455 ± .170 .98 ± .02

Una (46/50) 0 .749 ± .061 .96 ± .01

Ketengban (19/22)

0 .608 ± .127 .81 ± .07

Citak (28/39) .267 ± .107 .860 ± .056 .92 ± .03

Kayser et al. (2003): Am J Hum Genet 72:281-302

Social practices of Bantu-speaking groups?

• Can genetic studies inform us about the prehistoric social practices of Bantu-speaking groups (intermarriage with hunter-gathering populations, patrilocality)?

Upward social mobility

• Ethnographic assumption: agriculturalist men may marry hunter-gatherer women, but not vice versa

• Prediction: introgression of ‘hunter-gatherer mtDNA’ but not Y-chromosomes in agricultural populations

Upward social mobility

‘Khoisan-specific’ haplogroups

mtDNA L0d Y-chr. A-M51

Southern African Bantu speakers

Southern African Bantu-speakers

Upward social mobility

‘Khoisan-specific’ haplogroups

mtDNA L0d Y-chr. A-M51

Southern African Bantu speakers

4-7%

Southern African Bantu-speakers

Pereira et al. (2001): Ann Hum Genet 65: 439-458 Salas et al. (2002): Am J Hum Genet. 71: 1082–1111

Upward social mobility

‘Khoisan-specific’ haplogroups

mtDNA L0d Y-chr. A-M51

Southern African Bantu speakers

4-7%

Southern African Bantu-speakers

3-7%

Pereira et al. (2001): Ann Hum Genet 65: 439-458 Salas et al. (2002): Am J Hum Genet. 71: 1082–1111Wood et al. (2005): Eur J Hum Genet 13: 867-876

Upward social mobility

• Potentially intriguing finding:1) the social ideal is not always adhered to2) shift of language and identity of small groups of Khoisan-speakers in Southern Africa

Upward social mobility

‘Khoisan-specific’ haplogroups

mtDNA L0d Y-chr. A-M51

Southern African Bantu speakers

4-7%

Southern African Bantu-speakers

3-7%

Pereira et al. (2001): Ann Hum Genet 65: 439-458 Salas et al. (2002): Am J Hum Genet. 71: 1082–1111Wood et al. (2005): Eur J Hum Genet 13: 867-876

Upward social mobility

‘Khoisan-specific’ haplogroups

mtDNA L0d Y-chr. A-M51

Mozambique Bantu speakers

4-7% ??

South Africa Bantu-speakers

?? 3-7%

Pereira et al. (2001): Ann Hum Genet 65: 439-458 Salas et al. (2002): Am J Hum Genet. 71: 1082–1111Wood et al. (2005): Eur J Hum Genet 13: 867-876

Upward social mobility

• Caveat: groups not really comparable South African Nguni populations (Zulus, Xhosa) are known to have been in close contact with Khoisan speakers (‘borrowing’ of clicks)

Patrilocality in Bantu-speakers?

• Prediction: in patrilocal groups, mtDNA diversity should be higher than Y-chromosomal diversity

Haplogroup diversity values in some Bantu-speaking groups

Tishkoff et al. (2007): Mol Biol Evol 24: 2180-2195; Pereira et al. (2001): Ann Hum Genet 65: 439-458 Salas et al. (2002): Am J Hum Genet. 71: 1082–1111; Wood et al. (2005): Eur J Hum Genet 13: 867-876Quintana-Murci et al. (2008): PNAS 105: 1596–1601; De Filippo et al. unpublished

West CentralAfrica

NgumbaTuru

SukumaBisa

KundaSouthern

Africa

Y-chromosome

mtDNA

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Y-chromosome

mtDNA

Social practices of Bantu-speaking groups?

• Reduced Y-chromosomal diversity appears indicative of patrilocal post-marital residence

Data are not comparable

• ‘West Central Africa’ mtDNA = 20 groups from Cameroon and Gabon Y-chromosome = 3 groups from Cameroon

• ‘Southern Africa’mtDNA = ~ 20 different populations from Mozambique Y-chromosome = Sotho-Tswana, Zulu and Xhosa from South Africa

Data are not comparable

• practically no comparable data available for mtDNA and Y-chromosome in the same Bantu-speaking groups

Conclusions

• Genetic analyses can provide some insights into prehistoric social practices

• These may be of help for historical linguists in search of explanations for patterns of linguistic diversity (e.g. contact-induced change)

• However, comparable studies of both mtDNA and Y-chromosomal diversity in ethno-linguistically well-defined groups are needed

Acknowledgements

• Cesare de Filippo for Bisa and Kunda data• Mark Stoneking for discussion