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Most com­pre­hen­sive Tree of Life shows pla­cen­tal mam­mal diver­sity exploded after age of Dinosaurs

pub­lished 09 Feb­ru­ary 2013 | mod­i­fied 09 Feb­ru­ary 2013

Placental ancestorA ground­break­ing six-​year research col­lab­o­ra­tion has pro­duced the most com­plete pic­ture yet of the evo­lu­tion of pla­cen­tal mam­mals, the group that includes humans. Pla­cen­tal mam­mals are the largest branch of the mam­malian fam­ily tree, with more than 5,100 liv­ing species. Researchers from Carnegie Museum of Nat­ural His­tory are among the team of 23 that took part in this exten­sive inter­dis­ci­pli­nary effort that utilises mol­e­c­u­lar (DNA) and mor­pho­log­i­cal (anatomy) data on an extra­or­di­nary scale. By com­bin­ing these two types of data sci­en­tists recon­structed, to an unprece­dented level of detail, the fam­ily tree of pla­cen­tal mam­mals. This study explored thou­sands of char­ac­ter­is­tics of the anatomy of both liv­ing and extinct pla­cen­tal mam­mals.

This new project pro­duced a more com­plete pic­ture of mam­malian his­tory and pro­vides a huge dataset that will become the start­ing point of research for a num­ber of sci­en­tific ques­tions, includ­ing those of vital impor­tance today: how mam­mals may have sur­vived cli­mate change in the past and what may that mean for our future. The paper was pub­lished on Feb­ru­ary 8 in the jour­nal Sci­ence.

The col­lab­o­ra­tion is part of the Assem­bling the Tree of Life (ATOL) project funded by the National Sci­ence Foun­da­tion.

Join­ing forces, join­ing research
The recent arti­cle reveals the final results of the six-​year ATOL project. The study began with two teams organ­is­ing data from two dis­tinct approaches to evo­lu­tion­ary research: mol­e­c­u­lar data (DNA), and mor­pho­log­i­cal data (anatom­i­cal fea­tures).

In the field of mam­mal research, there had been a big divide between peo­ple work­ing with DNA and oth­ers work­ing on mor­phol­ogy,” explains John Wible, PhD, Cura­tor of Mam­mals at Carnegie Museum of Nat­ural His­tory and co-​author on the paper. “They just weren’t work­ing with each other until now.”

The mol­e­c­u­lar team col­lected DNA sequences of liv­ing ani­mals and the mor­phol­ogy team exam­ined the anatomy of both liv­ing and extinct mam­mals. The mol­e­c­u­lar team only sam­pled liv­ing mam­mals, because genetic mate­r­ial can’t be extracted from fos­sils older than 30,000 years. Thus, to include fos­sils, mor­pho­log­i­cal infor­ma­tion was essen­tial. Researchers in mor­phol­ogy deal with indi­vid­ual phys­i­cal fea­tures, from bone length to types of teeth to the pres­ence of stripes in the fur; each one of these fea­tures is termed a ‘char­ac­ter.’ By col­lect­ing as many char­ac­ters as pos­si­ble and com­par­ing their vari­a­tion among dozens of spec­i­mens, rela­tion­ships between species can be tested and broader pat­terns emerge.

The ATOL project became a mor­pho­log­i­cal pow­er­house. Gen­er­ally, a group of 500 char­ac­ters is con­sid­ered to be a large dataset. The mor­phol­ogy researchers on the ATOL project gen­er­ated an unprece­dented 4,500 char­ac­ters. Once both DNA and mor­pho­log­i­cal datasets were pro­duced, the result­ing com­bined matrix pro­vided an unprece­dented amount of infor­ma­tion for each of the 83 mam­mals included in the study.

It’s not that we hadn’t com­bined mor­phol­ogy with DNA before.” clar­i­fies co-​author Michelle Spauld­ing, PhD, the Rea Post-​doctoral Fel­low at Carnegie Museum of Nat­ural His­tory. “This time, we ratch­eted up the amount of mor­pho­log­i­cal detail phe­nom­e­nally, pro­vid­ing a larger anatom­i­cal base for the study as com­pared with DNA than is typ­i­cal.”

I think this data­base is amaz­ing because it’s being pre­sented in such a way that it will be repro­ducible for the future generations
Jonathan Bloch, co-​author, asso­ciate cura­tor of ver­te­brate pale­on­tol­ogy at the Florida Museum of Nat­ural His­tory »

With the new Tree of Life matrix, researchers now have greater con­text for the frag­men­tary fos­sils they have in hand — often scant evi­dence such as a few teeth or a skull frag­ment — poten­tially shed­ding light on little-​known species that have yet to find a solid home in the evo­lu­tion­ary tree.

Ances­tral ori­gins esti­mated
Thanks to the incred­i­ble amount of anatom­i­cal infor­ma­tion col­lected, the researchers were able to pre­dict the appear­ance of the most recent com­mon ances­tor of all pla­cen­tal mam­mals. Explains Spauld­ing:

We have all these pla­cen­tals alive today, from ele­phants to shrews, from things that fly to things that swim. What could the com­mon ances­tor of these things that are so dif­fer­ent pos­si­bly look like?

That’s the power of 4,500 char­ac­ters,” says Wible. “We looked at all aspects of mam­malian anatomy, from the skull and skele­ton, to the teeth, to inter­nal organs, to mus­cles, and even fur pat­terns. Using the new fam­ily tree of mam­mals in tan­dem with this anatom­i­cal data, we were able to recon­struct what this com­mon pla­cen­tal ances­tor may have looked like.”

The sci­en­tists were able to work with an artist to approx­i­mate the appear­ance of this ances­tor. While only hypo­thet­i­cal, the illus­trated con­cept for this ances­tor — from body size to fur type to num­ber of teeth — could not have been achieved prior to the Her­culean task of devel­op­ing the matrix.

Ear­li­est date for pla­cen­tals
We focused our study on the time around the Cretaceous-​Tertiary (KT) bound­ary, 65 mil­lion years ago,” states Spauld­ing, “Mol­e­c­u­lar and mor­pho­log­i­cal based stud­ies dif­fer on the age when pla­cen­tals first appeared. Mol­e­c­u­lar stud­ies place the ori­gin in the mid-​Cretaceous, when dinosaurs still dom­i­nated. On the other hand, mor­pho­log­i­cal stud­ies have rou­tinely found no evi­dence of any pla­cen­tal fos­sils in this time period, and instead place the pla­cen­tal ori­gin after the mass extinc­tion at the close of the Cre­ta­ceous that ended dinosaur dominance.”

One major goal of this project was to address this con­tro­versy and results found that pla­cen­tal mam­mals appeared after the KT bound­ary, imply­ing that the mass extinc­tion [that ended dinosaur dom­i­nance, Moos] was a crit­i­cal event in mam­malian evo­lu­tion­ary history.
New web­site an essen­tial tool
The study was con­ducted util­is­ing the web appli­ca­tion Mor­phobank. The matrix is freely avail­able online and pro­vides a road map to the Tree of Life team’s find­ings by pre­cisely out­lin­ing how the team defined each of the more than 4,500 char­ac­ters in the dataset. Prov­ing that a pic­ture is worth a thou­sand words, the major­ity of char­ac­ters include illus­tra­tions.

As rev­o­lu­tion­ary as the study, the web­site also marks a new era in how col­lab­o­ra­tive research may pro­ceed. “We couldn’t have accom­plished this with­out Mor­phobank,” lauds Spauld­ing. “This web­site allowed mem­bers of the team, spread all over the globe, to work simul­ta­ne­ously.”

The ATOL project
The Assem­bling the Tree of Life (ATOL) project, funded by the National Sci­ence Foun­da­tion, is an effort to pro­mote inter­dis­ci­pli­nary part­ner­ships in global bio­log­i­cal research. The ATOL pro­gram encour­ages researchers of dif­fer­ent top­ics — such as fos­sils, bio­di­ver­sity, genet­ics, and evo­lu­tion — to com­bine their inde­pen­dent lines of study into a sin­gle large frame­work, or matrix. This matrix con­tex­tu­alises their indi­vid­ual research find­ings and is a tool for con­firm­ing exist­ing evo­lu­tion­ary rela­tion­ships and pos­si­bly dis­cov­er­ing new ones in light of pre­vi­ously unrecog­nised con­nec­tions.

The above news item is reprinted from mate­ri­als avail­able at Carnegie Museum of Nat­ural His­tory and Uni­ver­sity of Florida. Orig­i­nal text may be edited for con­tent and length.
(Source: Carnegie Museum of Nat­ural His­tory press release, 07.02.2013; Uni­ver­sity of Florida News, 07.02.2013)

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