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Light human skeleton may have come after agriculture

Dec. 23, 2014
Courtesy of PNAS
and World Science staff

The rel­a­tively lightly built skele­tons of mod­ern hu­mans arose late in our ev­o­lu­tion­ary his­to­ry, and may have re­sulted from a less ac­tive lifestyle af­ter ag­ri­cul­ture spread, re­search­ers re­port.

Re­cent mod­ern hu­mans have a lightly built skel­e­ton, com­pared with those of chim­panzees and ex­tinct hu­man spe­cies.

In a new stu­dy, Habiba Chirchir of the The George Wash­ing­ton Uni­vers­ity in Wash­ing­ton, D.C. and col­leagues ex­am­ined a type of hu­man bone tis­sue called tra­bec­u­lar bone, which is rel­a­tively light and spon­gy in struc­ture. Tra­bec­u­lar bone tis­sue co-exists with­in the same bones as the more com­pact type of bone tis­sue, called com­pact or cor­ti­cal bone.

The re­search­ers stud­ied “spon­gy” bone through­out the ske­l­e­ton of mod­ern hu­mans and chimps, as well as in fos­sils of ex­tinct hu­man spe­cies span­ning sev­er­al mil­lion years. The in­ves­ti­ga­tors found that the up­per and low­er limbs of re­cent mod­ern hu­mans are lightly built com­pared with those of the oth­er groups. 

The change al­so ap­pears to have hap­pened rel­a­tively ab­rupt­ly, the sci­en­tists added. The de­crease in bone dens­ity was found to be more marked in low­er limbs than in up­per limbs, sug­gest­ing changes in mo­bil­ity as a pos­si­ble cause, they said.

The find­ings are pub­lished this week in the re­search jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces

In a re­lat­ed ar­ti­cle pub­lished in the same is­sue of the jour­nal, Tim­o­thy M. Ryan of Penn­syl­va­nia State Uni­vers­ity and Col­in N. Shaw of Cam­bridge Uni­vers­ity as­sessed how be­hav­ior­al pat­terns af­fect the skel­e­ton. They com­pared hip joints among four ar­che­o­logical hu­man popula­t­ion­s—rep­re­senting mo­bile for­agers and sed­en­tary agri­cul­tur­alists a large sam­ple of ex­ist­ing pri­ma­tes. 

Ryan and Shaw found that the highly mo­bile for­agers had sig­nif­i­cantly thicker and stronger bones in their hip joints, com­pared with the agri­cul­tur­alists, and the bone strength and struc­ture of for­agers’ hip joints was com­pa­ra­ble to that of si­m­i­larly sized non-hu­man pri­ma­tes. 

The dif­fer­ences sug­gest that phys­i­cal ac­ti­vity may in­flu­ence bone mass in the hip joint, show­ing a link be­tween re­duced phys­i­cal ac­ti­vity and de­creased bone strength, the au­thors said. They added that ex­er­cise may re­main im­por­tant for bone health and the re­duc­tion of age-re­lat­ed bone loss, os­te­o­por­osis and frac­ture risk.


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The relatively lightly built skeletons of modern humans arose late in our evolutionary history, and may have resulted from a less active lifestyle after agriculture spread, researchers report. Recent modern humans have a lightly built skeleton, compared with those of chimpanzees and extinct human species. In a new study, Habiba Chirchir of the The George Washington University in Washington, D.C. and colleagues examined a type of human bone tissue called trabecular bone, which is relatively light and spongy in structure. Trabecular bone tissue co-exists within the same bones as the more compact type of bone tissue, called compact o cortical bone. The researchers studied “spongy” bone throughout the skeleton of modern humans and chimps, as well as in fossils of extinct human species spanning several million years. The investigators found that the upper and lower limbs of recent modern humans are lightly built compared with those of the other groups. The change also appears to have happened relatively abruptly, the scientists added. The decrease in bone density was found to be more marked in lower limbs than in upper limbs, suggesting changes in mobility as a possible cause, they said. The findings are published this week in the research journal pnas. In a related article published in the same issue of the journal, Timothy M. Ryan of Pennsylvania State University and Colin N. Shaw of Cambridge University assessed how behavioral patterns affect the skeleton. They compared the bone architecture of the human hip joint among four distinct archeological human populations—representing mobile foragers and sedentary agriculturalists—and a large sample of existing primates. Ryan and Shaw found that the highly mobile foragers had significantly thicker and stronger bones in their hip joints, compared with the agriculturalists, and the bone strength and structure of foragers’ hip joints was comparable to that of similarly sized non-human primates. The differences suggest that physical activity may influence bone mass in the hip joint, showing a link between reduced physical activity and decreased bone strength, the authors said. They added that exercise may remain important for bone health and the reduction of age-related bone loss, osteoporosis, and fracture risk.