SAMPLINGS 



Made in India 



A dizzying variety of cultures and languages 

 flourish among India's billion-plus residents. 

 Did the differences arise among the descen- 

 dants of that nation's first settlers, who like- 

 ly arrived in South Asia from Africa more 

 than 40,000 years ago, or do they 

 reflect subsequent waves of immigration? 

 Northern people often share cultural prac- 

 tices such as farming, social castes, and 

 Indo-European languages, which has 

 prompted speculation that their ancestors 

 immigrated in a more recent wave, possibly 

 from West or Central Asia. 



But several studies have shown scant vari- 

 ation in the mitochondrial DNA of Indians 

 throughout the nation, and little similarity to 

 populations outside South Asia. That sug- 

 gests a single, early origin. Mitochondrial 

 DNA is passed only from mother to child, 

 however, and so, strictly speaking, what the 

 studies have shown is that only one wave of 

 female immigrants entered prehistoric India. 

 The DNA of the Y chromosome, passed 

 from father to son, can help show whether 

 there was an influx of men. 



A team of geneticists led by Sanghami- 

 tra Sahoo and V.K. Kashyap from the Na- 



At a crux in the family tree 



Indian men: the Ys that bind? 



tional DNA Analysis Centre in Calcutta 

 examined the Y chromosomes of men 

 throughout India. They, too, found little 

 genetic evidence for a second wave of im- 

 migrants to India. Only one small group, 

 Tibeto-Burman-language speakers in the 

 northeast, seemed to have arrived rela- 

 tively recently, probably from East Asia. 

 The Indo-European-language speakers, 

 by contrast, appear to be native born. By 

 and large, then, India's cultural differences 

 probably evolved within a somewhat ge- 

 netically isolated population. {PNAS 

 103:843-8,2006) —SR. 



Time Dilation 



Like every other living thing, we humans 

 and our nearest relatives, the chim- 

 panzees, have "junk" DNA. It probably 

 doesn't code for anything functional, but 

 it sure is useful to evolutionary biologists. 

 Because mutations within noncoding DNA 

 are not exposed to the rigors of natural 

 selection, they accumulate. And because 

 they tend to arise at regular intervals, 

 they are useful as "molecular clocks." 

 Knowing the mutation rate and the num- 

 ber of genetic differences between two 

 species, evolutionary biologists can esti- 

 mate when the species diverged: some 6 

 million years ago, in the case of humans 

 and chimpanzees. 



In the geologically recent past, how- 

 ever, human generations have been longer 

 than those of our cousins: about twenty 

 years, on average, compared to the chim- 

 panzee's fifteen. Now a team of geneti- 



Ocean Genome 



Microscopic life thrives in the open ocean, 

 where it plays a key role in the complex flux 

 of matter and energy. Yet its ecology re- 

 mains poorly understood. 



At the ALOHA oceanographic station, 

 sixty miles north of the island of Oahu, in 

 Hawai i, microbial oceanographers Ed- 

 ward F. DeLong of the Massachusetts In- 

 stitute of Technology in Cambridge and 

 David M. Karl of the University of Hawai'i 

 in Honolulu, and several colleagues sam- 

 pled microorganisms at depths ranging 

 from 32 to 13,000 feet. Their plan was to 

 analyze how the gene sequences varied 

 by depth, taking into account the physi- 

 cal, chemical, and other biological prop- 

 erties of the water column, as measured 

 by ALOHA. 



Sure enough, DeLong and his colleagues 

 detected predictable trends in gene func- 

 tion associated with the distinct microbial 

 communities they found living at various 

 depths. Genes involved in metabolism from 

 sunny surface waters, for instance, often 

 code for photosynthetic pathways, whereas 

 in deep waters, genes for metabolizing 

 methane and other sources of chemical en- 

 ergy predominate. From shallow waters, the 

 investigators also recovered a surprising 

 number of virus genomes that had incorpo- 

 rated genes involved in photosynthesis from 

 their cyanobacteria hosts. In the genetic 

 bazaar of the sea, such gene exchange 

 among microorganisms may be quite wide- 

 spread. (Science 31 1 :496-503, 2006) 



— Graciela Flores 



cists has determined that the difference 

 in generation span has led to different 

 rates of mutation in noncoding DNA. 



Navin Elango and Soojin V. Yi, both 

 geneticists at the Georgia Institute of 

 Technology in Atlanta, and their col- 

 leagues conducted one of the largest 

 and most precise comparisons ever 

 made between the noncoding DNA of 

 humans and our closest relatives. Ac- 

 cording to their findings, the human mol- 

 ecular clock does indeed tick slower than 

 that of chimpanzees, which in turn runs 

 slower than the clocks of gorillas and 

 orangutans. Yet the differences are so 

 small that longer generations among hu- 

 mans likely evolved just a million years 

 ago. {PNAS 103:1370-5, 2006) —S.R. 



NATURAL HISTORY April 2006 



