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What howler mon­keys can tell us about the role of inter­breed­ing in human evolution

201208Dec22:17

What howler mon­keys can tell us about the role of inter­breed­ing in human evolution

Infor­ma­tion
pub­lished 08 Decem­ber 2012 | mod­i­fied 08 Decem­ber 2012
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Did dif­fer­ent species of early humans inter­breed and pro­duce off­spring of mixed ances­try?

Howler monkeyRecent genetic stud­ies sug­gest that Nean­derthals may have bred with anatom­i­cally mod­ern humans tens of thou­sands of years ago in the Mid­dle East, con­tribut­ing to the mod­ern human gene pool. But the find­ings are not uni­ver­sally accepted, and the fos­sil record has not helped to clar­ify the role of inter­breed­ing, which is also known as hybridi­s­a­tion.

Now a Uni­ver­sity of Michi­gan (UM)-led study of inter­breed­ing between two species of modern-​day howler mon­keys in Mex­ico is shed­ding light on why it’s so dif­fi­cult to con­firm instances of hybridi­s­a­tion among pri­mates — includ­ing early humans — by rely­ing on fos­sil remains.

The study, pub­lished online Decem­ber 7 in the Amer­i­can Jour­nal of Phys­i­cal Anthro­pol­ogy, is based on analy­ses of genetic and mor­pho­log­i­cal data col­lected from live-​captured mon­keys over the past decade. Mor­phol­ogy is the branch of biol­ogy that deals with the form and struc­ture of ani­mals and plants.

The two pri­mate species in the study, man­tled howler mon­keys (Alouatta pal­li­ata) and black howler mon­keys (Alouatta pigra), diverged about 3 mil­lion years ago and dif­fer in many respects, includ­ing behav­iour, appear­ance and the num­ber of chro­mo­somes they pos­sess. Each occu­pies a unique geo­graph­i­cal dis­tri­b­u­tion except for the state of Tabasco in south­east­ern Mex­ico, where they coex­ist and inter­breed in what’s known as a hybrid zone.

Howler mon­keys are among the largest of New World mon­keys, with male man­tled howlers weigh­ing up to 22 pounds. Four­teen species of howler mon­keys are cur­rently recog­nised. They are native to Cen­tral and South Amer­i­can forests, in addi­tion to south­east­ern Mexico.
The researchers found that indi­vid­u­als of mixed ances­try who share most of their genome with one of the two species are phys­i­cally indis­tin­guish­able from the pure indi­vid­u­als of that species.

The impli­ca­tions of these results are that phys­i­cal fea­tures are not always reli­able for iden­ti­fy­ing indi­vid­u­als of hybrid ances­try. There­fore, it is pos­si­ble that hybridi­s­a­tion has been under­es­ti­mated in the human fos­sil record
Lil­iana Cortés-​Ortiz, evo­lu­tion­ary biol­o­gist and pri­ma­tol­o­gist, UM Depart­ment of Ecol­ogy and Evo­lu­tion­ary Biol­ogy and the Museum of Zool­ogy »

For years, anthro­pol­o­gists have attempted to infer hybridi­s­a­tion among human ances­tral species based on the fos­sil record, which rep­re­sents only a snap­shot in pre­his­tory, and have con­cluded that hybridi­s­a­tion is extremely rare, accord­ing to lead author Mary Kelaita and Lil­iana Cortés-​Ortiz. Given the util­ity of liv­ing pri­mate mod­els for under­stand­ing human evo­lu­tion, the howler mon­key study “sug­gests that the lack of strong evi­dence for hybridi­s­a­tion in the fos­sil record does not negate the role it could have played in shap­ing early human lin­eage diver­sity,” Kelaita said.

The authors con­clude that the process of hybridi­s­a­tion (defined as the pro­duc­tion of off­spring through the inter­breed­ing between indi­vid­u­als of genet­i­cally dis­tinct pop­u­la­tions), the fac­tors gov­ern­ing the expres­sion of mor­phol­ogy in hybrid indi­vid­u­als, and the extent of repro­duc­tive iso­la­tion between species should be given fur­ther con­sid­er­a­tion in future research projects.

In their study between 1998 and 2008, Kelaita and Cortés-​Ortiz, sam­pled 135 adult howler mon­keys from Tabasco, Mex­ico, along with 76 oth­ers from Ver­acruz, Campeche, Chi­a­pas and Quin­tana Roo states in Mex­ico and Peten in Guatemala. They analysed dif­fer­ent types of genetic mark­ers, from both mito­chon­dr­ial and nuclear DNA, to trace the ances­try of each howler mon­key they stud­ied. The use of mol­e­c­u­lar mark­ers made it pos­si­ble to approx­i­mate the rel­a­tive genetic con­tri­bu­tions of the parental species to each hybrid. A total of 128 hybrid indi­vid­u­als were detected, and most were likely the prod­uct of sev­eral gen­er­a­tions of hybridi­s­a­tion or of mat­ing between hybrids and pure indi­vid­u­als.

Sub­se­quently, a sta­tis­ti­cal analy­ses on body mea­sure­ments showed a large amount of mor­pho­log­i­cal vari­a­tion in indi­vid­u­als of mixed ances­try. How­ever, when indi­vid­u­als were clas­si­fied accord­ing to the amount of their genome they shared with each parental species, it became clear that indi­vid­u­als of mixed ances­try that shared most of their genome with one of the species were phys­i­cally indis­tin­guish­able from the pure indi­vid­u­als of that species. Even indi­vid­u­als that were more “inter­me­di­ate” in their genetic com­po­si­tion were not com­pletely inter­me­di­ate in their appear­ance.
The study is the first to assess genetic ances­try of pri­mate hybrids inhab­it­ing a nat­ural hybrid zone using mol­e­c­u­lar data to explain mor­pho­log­i­cal variation.


The above news item is reprinted from mate­ri­als avail­able at Uni­ver­sity of Michi­gan. Orig­i­nal text may be edited for con­tent and length.
(Source: Uni­ver­sity of Michi­gan news release, 07.12.2012)
Tags: evo­lu­tion | hybridi­s­a­tion | howler mon­key | human | Mex­ico
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