Modern horses are expert runners. They reach top speeds using a special running gait in which they hold their back stiff as they move. A new study published today reveals that tiny fossil ancestors of modern horses may have moved quite differently to their living counterparts.
"Horses provide a perfect case-study on the evolution of running because they have such an amazing fossil record", explains author Dr. Katrina Jones, a post-doctoral researcher in Harvard's Museum of Comparative Zoology. Dating back over 50 million years, the oldest horse ancestors were no bigger than a house cat. From those ancient horse ancestors, some lineages evolved larger sizes, grazing habit and limbs that were specialized for running. This new study suggests that the stiff-backed gait of modern horses likely evolved to save energy while running as horses got bigger through their evolution.
"For over a century, researchers studied the feet of fossil horses to explain how they evolved features specialized for running," explains Jones, "but very little is known about how the backbone might be involved in this famous transition." Four-legged mammals tend to move their lower back during running to help increase speed and regulate breathing. But horses are unusual because they restrict the motion of their lumbar spine to a single joint near their rump. Jones wanted to find out if this unusual pattern was shared by extinct horses, and how increasing size in horse evolution may have affected their back mobility and running style.
To understand the evolution of the back in fossil horses, Jones first examined the anatomy and mobility of the spine in modern domestic horses. The shape of the vertebral joints--bony connections between the vertebrae--help determine how much motion occurs at each joint. Armed with this information, Jones then measured the shape of vertebral joints in 16 species of fossil horses spanning their full size and age range.
The inner ear of Protungulatum (Pan-Euungulata, Mammalia)
Authors:
Orliac et al
Abstract:
We present new anatomical details about the bony labyrinth of Protungulatum based on micro CT-scan investigation of an isolated petrosal bone retrieved at the Puercan locality of Bug Creek Anthills and referred to Protungulatum sp. The exceptional state of preservation of the specimen allowed us to reconstruct the very fine details of the inside of the petrosal bone, including the bony labyrinth, the innervation of the vestibule and the innervation and vasculature of the cochlea. Estimation of the auditory capability of Protungulatum based on cochlear morphology indicate that Protungulatum was specialized for high-frequency hearing, with estimated low frequency limits above 1 KHz. Comparisons with Late Cretaceous non-placental eutherians and with early Tertiary pan-euungulates indicate that the bony labyrinth of Protungulatum is closer in general morphology to Mesozoic forms (low coiling and low aspect ratio of the cochlea, posterior orientation of the common crus, dorsal outpocketing of the cochlear fossula), and shares only a few characters with pan-euungulate and euungulate taxa. Interestingly, the bony labyrinth of Protungulatum also shares some morphological features with South American notoungulates and litopterns recently described from Itaboraí, Brazil. These new observations provide new morphological features of potential phylogenetic interest.
The subtalar joint complex of Australopithecus sediba
Authors:
Prang et al
Abstract:
The hominin talus has figured prominently in previous studies of the functional morphology of the talocrural joint, but the talocalcaneal and talonavicular joints have received comparatively less attention despite their functional importance as components of the subtalar joint complex. An associated complete talus and calcaneus attributed to the Malapa Hominin 2 (MH2) individual of Australopithecus sediba offers the opportunity to evaluate the subtalar joint complex in an early hominin. Furthermore, detailed morphological comparisons of A. sediba to other fossil hominins such as Australopithecus africanus have not yet been conducted. Here I quantify joint curvatures and angular measurements among extant hominoids and fossil hominins to evaluate the functional morphology of the subtalar joint complex of A. sediba. Australopithecus sediba uniquely combines talocalcaneal joint morphology indicative of mobility with specializations of the talonavicular joint that provide medial midtarsal stabilization. Multivariate analyses of talus and calcaneus variables show that A. sediba is most similar to extant gorillas in the morphology of the subtalar joint complex. In contrast, other hominins, such as OH 8, are more similar to modern humans. The morphological similarity between MH2 (U.W. 88-98/99) and specimens from Sterkfontein, Member 4 (StW 88, StW 102, StW 352) in morphologies of the talonavicular and talocalcaneal joints suggests that A. sediba may have possessed a foot that was functionally similar to that of A. africanus. This combination of morphologies in the A. sediba foot is probably derived among hominins and suggests that arboreality may have been adaptively significant for southern African Australopithecus.
Cranial vault thickness in primates: Homo erectus does not have uniquely thick vault bones
Authors:
Copes et al
Abstract:
Extremely thick cranial vaults have been noted as a diagnostic characteristic of Homo erectus since the first fossil of the species was identified, but relatively little work has been done on elucidating its etiology or variation across fossils, living humans, or extant non-human primates. Cranial vault thickness (CVT) is not a monolithic trait, and the responsiveness of its layers to environmental stimuli is unknown.
We obtained measurements of cranial vault thickness in fossil hominins from the literature and supplemented those data with additional measurements taken on African fossil specimens. Total CVT and the thickness of the cortical and diploë layers individually were compared to measures of CVT in extant species measured from more than 500 CT scans of human and non-human primates.
Frontal and parietal CVT in fossil primates was compared to a regression of CVT on cranial capacity calculated for extant species. Even after controlling for cranial capacity, African and Asian H. erectus do not have uniquely high frontal or parietal thickness residuals, either among hominins or extant primates. Extant primates with residual CVT thickness similar to or exceeding H. erectus (depending on the sex and bone analyzed) include Nycticebus coucang, Perodicticus potto, Alouatta caraya, Lophocebus albigena, Galago alleni, Mandrillus sphinx, and Propithecus diadema. However, the especially thick vaults of extant non-human primates that overlap with H. erectus values are composed primarily of cortical bone, while H. erectus and other hominins have diploë-dominated vault bones. Thus, the combination of thick vaults comprised of a thickened diploë layer may be a reliable autapomorphy for members of the genus Homo.
Cochlear labyrinth volume in Krapina Neandertals
Authors:
Beals et al
Abstract:
Research with extant primate taxa suggests that cochlear labyrinth volume is functionally related to the range of audible frequencies. Specifically, cochlear volume is negatively correlated with both the high and low frequency limits of hearing so that the smaller the cochlea, the higher the normal range of audible frequencies. The close anatomical relationship between the membranous cochlea and the bony cochlear labyrinth allows for the determination of cochlear size from fossil specimens. This study compares Krapina Neandertal cochlear volumes to extant taxa cochlear volumes. Cochlear volumes were acquired from high-resolution computed tomography scans of temporal bones of Krapina Neandertals, chimpanzees, gorillas, and modern humans. We find that Krapina Neandertals' cochlear volumes are similar to modern Homo sapiens and are significantly larger than chimpanzee and gorilla cochlear volumes. The measured cochlear volume in Krapina Neandertals suggests they had a range of audible frequencies similar to the modern human range.
Fossil hominin radii from the Sima de los Huesos Middle Pleistocene site (Sierra de Atapuerca, Spain)
Authors:
Rodriguez et al
Abstract:
Complete radii in the fossil record preceding recent humans and Neandertals are very scarce. Here we introduce the radial remains recovered from the Sima de los Huesos (SH) site in the Sierra de Atapuerca between 1976 and 2011 and which have been dated in excess of 430 ky (thousands of years) ago. The sample comprises 89 specimens, 49 of which are attributed to adults representing a minimum of seven individuals. All elements are described anatomically and metrically, and compared with other fossil hominins and recent humans in order to examine the phylogenetic polarity of certain radial features. Radial remains from SH have some traits that differentiate them from those of recent humans and make them more similar to Neandertals, including strongly curved shafts, anteroposterior expanded radial heads and both absolutely and relatively long necks. In contrast, the SH sample differs from Neandertals in showing a high overall gracility as well as a high frequency (80%) of an anteriorly oriented radial tuberosity. Thus, like the cranial and dental remains from the SH site, characteristic Neandertal radial morphology is not present fully in the SH radii. We also analyzed the cross-sectional properties of the SH radial sample at two different levels: mid-shaft and at the midpoint of the neck length. When standardized by shaft length, no difference in the mid-shaft cross-sectional properties were found between the SH hominins, Neandertals and recent humans. Nevertheless, due to their long neck length, the SH hominins show a higher lever efficiency than either Neandertals or recent humans. Functionally, the SH radial morphology is consistent with more efficient pronation-supination and flexion-extension movements. The particular trait composition in the SH sample and Neandertals resembles more closely morphology evident in recent human males.
Physiological implications of the abnormal absence of the parietal foramen in a late Permian cynodont (Therapsida)
Authors:
Benoit et al
Abstract:
The third eye (pineal eye), an organ responsible for regulating exposure to sunlight in extant ectotherms, is located in an opening on the dorsal surface of the skull, the parietal foramen. The parietal foramen is absent in extant mammals but often observed in basal therapsids, the stem-group to true mammals. Here, we report the absence of the parietal foramen in a specimen of Cynosaurus suppostus, a Late Permian cynodont from South Africa (SA). Comparison with Procynosuchus delaharpeae, a contemporaneous non-mammalian cynodont from SA, demonstrates that the absence of this foramen is an abnormal condition for such a basal species. Because seasonality was marked during the Late Permian in SA, it is proposed that the third eye was functionally redundant in Cynosaurus, possibly due to the acquisition of better thermoregulation or the evolution of specialized cells in the lateral eyes to compensate for the role of the third eye.
The Neanderthals (Homo neanderthalensis) inhabited Europe and parts of western Asia between 230,000 and 28,000 years ago; during the last few millennia they coincided with Homo Sapiens Sapiens, and became extinct for reasons that are still being challenged. The archaeological site at La Ferrassie, excavated throughout the 20th century, is a mythical enclave because it was where 7 Neanderthal skeletons, ranging from foetuses to almost complete skeletons of adults, were found.
Among the remains discovered at La Ferrassie is the skeleton of a 2-year-old Neanderthal child found between 1970 and 1973 and baptised La Ferrassie 8; over 40 years since its discovery it has turned out to be useful in shedding new light on the anatomy of this extinct species.
The study began by reviewing the collections at the Muséum National d'Histoire Naturelle in Paris and at the Museo d'Archéologie national de St. Germain-en-Laye linked to the excavations at La Ferrassie in 1970 and 1973; it was there that 47 new fossils belonging to La Ferrassie 8, which complete its skeleton further, were recovered. Remains of a skull, jaw, vertebrae, ribs and hand phalanges were found among the new fossils.
Featuring among the remains is a very complete left temporal bone and an auditory ossicle was found inside it: a complete stapes. Virtual 3D reconstruction techniques enabled this ossicle to be "extracted virtually" and studied.
This stapes is the most complete one in the Neanderthal record and certifies that there are morphological differences between our species and the Neanderthals even in the smallest ossicles in the human body. As Asier Gómez-Olivencia pointed out, "we do not yet know the relation between these morphological differences and hearing in the Neanderthals. This would constitute a new challenge for the future".
Calcaneal robusticity in Plio-Pleistocene hominins: Implications for locomotor diversity and phylogeny
Author:
Prang
Abstract:
A key pedal adaptation to bipedality is a relatively large, weight-bearing calcaneus. The earliest evidence for a human-like, robust calcaneus is at 3.2 Ma in Australopithecus afarensis (A.L. 333-8, A.L. 333-55, A.L. 333-37) from Hadar, Ethiopia. Australopithecus sediba at 1.98 Ma from Malapa, South Africa displays a unique combination of primitive australopith features and more derived Homo-like features, but surprisingly is characterized by a gracile, chimpanzee-like calcaneus. The differences in calcaneal morphology suggest that these taxa differed in the frequency of arboreality and in the manner of foot function during terrestrial bipedal locomotion. This study examines calcaneal morphology in extant hominids (i.e., great apes and humans; N = 95) and fossil hominins (N = 5) to better understand the evolutionary development of calcaneal robusticity in early hominins. In particular, this study focuses on two additional fossil hominin calcanei that have not figured prominently in previous discussions of calcaneal robusticity: StW 352 and Omo 33-74-896. A measure of calcaneal robusticity was quantified as the ratio of calcaneal tuber cross-sectional area to calcaneal tuber length, which significantly differs between humans and non-humans using a sequential Bonferroni alpha adjustment for multiple comparisons. Additional multivariate analyses using Mosimann shape variables show that StW 352 and Omo 33-74-896 are more similar to Au. sediba in calcaneal tuber morphology than to Au. afarensis, suggesting that the latter taxon is better adapted for terrestrial bipedalism than at least some later species of Australopithecus. This finding implies the possibility of several complex evolutionary scenarios involving either multiple reversals in postcranial morphology in Australopithecus or the independent acquisition of adaptations to terrestrial bipedalism in Au. afarensis and Homo.
Human-like hand use in Australopithecus africanus
Authors:
Skinner et al
Abstract:
The distinctly human ability for forceful precision and power “squeeze” gripping is linked to two key evolutionary transitions in hand use: a reduction in arboreal climbing and the manufacture and use of tools. However, it is unclear when these locomotory and manipulative transitions occurred. Here we show that Australopithecus africanus (~3 to 2 million years ago) and several Pleistocene hominins, traditionally considered not to have engaged in habitual tool manufacture, have a human-like trabecular bone pattern in the metacarpals consistent with forceful opposition of the thumb and fingers typically adopted during tool use. These results support archaeological evidence for stone tool use in australopiths and provide morphological evidence that Pliocene hominins achieved human-like hand postures much earlier and more frequently than previously considered.
pop sci
write up.
Range of Movement in Ray I of Manus and Pes and the Prehensility of the Autopodia in the Early Permian to Late Cretaceous Non-Anomodont Synapsida
Authors:
Kümmell et al
Abstract:
The mobility of ray I was analysed in seventy-eight Early Permian to Late Cretaceous specimens of non-mammalian Synapsida and one extant mammal. In all non-mammaliamorph Synapsida investigated, ray I formed a digital arcade. The first phalanx was maximally extendable to the zero position in the metapodiophalangeal joint I. Metapodiale I was the functional equivalent to a basal phalanx of digits II–V. In contrast, there was no digital arcade in ray I in Mesozoic Mammaliamorpha. Phalanx 1 I was dorsally extendable and metapodiale I was functionally part of the metapodium. During the propulsion phase, autopodial rotation occurred in the majority of Synapsida with abducted limb posture. Regarding ray I, the reduction of autopodial rotation can be estimated, e.g., from the decrease of lateral rotation and medial abduction of the first phalanx in the metapodiophalangeal joint I. Autopodial rotation was high in Titanophoneus and reduced in derived Cynodontia. In Mammaliamorpha the mobility of the first ray suggests autopodial rolling in an approximately anterior direction. Most non-mammaliamorph Therapsida and probably some Mesozoic Mammaliamorpha had prehensile autopodia with an opposable ray I. In forms with a pronounced relief of the respective joints, ray I could be opposed to 90° against ray III. A strong transverse arch in the row of distalia supported the opposition movement of ray I and resulted in a convergence of the claws of digits II–V just by flexing those digits. A tight articular coherence in the digital joints of digits II–V during strong flexion supported a firm grip capacity. Usually the grip capacity was more pronounced in the manus than in the pes. Prehensile autopodia of carnivorous Therapsida may have been utilized to hold prey while biting, thus helping to avoid fractures of the laterally compressed fangs.
New research shows that modern human skeletons evolved into their lightly built form only relatively recently--after the start of the Holocene about 12,000 years ago and even more recently in some human populations. The work, based on high-resolution imaging of bone joints from modern humans and chimpanzees as well as from fossils of extinct human species shows that for millions of years extinct humans had high bone density until a dramatic decrease in recent modern humans. Published this week in the Proceedings of the National Academy of Sciences, the findings reveal a higher decrease in the density of lower limbs than in that of the upper limbs, suggesting that the transformation may be linked to humans' shift from a foraging lifestyle to a sedentary agricultural one.
"Despite centuries of research on the human skeleton, this is the first study to show that human skeletons have substantially lower density in joints throughout the skeleton, even in ancient farmers who actively worked the land," said Brian Richmond, an author of the study, curator in the American Museum of Natural History's Division of Anthropology, and a research professor at George Washington University.
Compared to our closest living relatives--chimpanzees--as well as to our extinct human ancestors, humans are unique in having an enlarged body size and lower-limb joint surfaces in combination with a relatively lightweight skeleton. But until now, scientists did not know that human bone joints are significantly less dense compared with those of other animals, or when during human evolution this unique characteristic first appeared.
"Our study shows that modern humans have less bone density than seen in related species, and it doesn't matter if we look at bones from people who lived in an industrial society or agriculturalist populations that had a more active life. They both have much less bone density," said Habiba Chirchir, lead author of the paper and a postdoctoral researcher at the Smithsonian Institution's National Museum of Natural History, who started the work at George Washington University with Richmond. "What we want to know now is whether this is an early human characteristic that defines our species."
To explore this question, Chirchir, Richmond, and an international team of researchers used high-resolution computed tomography and microtomography to measure trabecular, or spongy, bone of the limb joints in modern humans and chimpanzees, as well as in fossil hominins attributed to Australopithecus africanus, Paranthropus robustus, Homo neanderthalensis, and early Homo sapiens. Their results show that only recent modern humans have low trabecular density throughout limb joints, and that the decrease is especially pronounced in the lower joints--those in the hip, knee, and ankle--rather than the upper joints in the shoulder, elbow, and hand. The appearance of this anatomical change late in our evolutionary history may have been a result of the transition from a nomadic to a more settled lifestyle.
"Much to our surprise, throughout our deep past, we see that our human ancestors and relatives, who lived in natural settings, had very dense bone. And even early members of our species, going back 20,000 years or so, had bone that was about as dense as seen in other modern species," Richmond said. "But this density drastically drops off in more recent times, when we started to use agricultural tools to grow food and settle in one place."
New Postcranial Material of the Early Caseid Casea broilii Williston, 1910 (Synapsida: Caseidae) with a Review of the Evolution of the Sacrum in Paleozoic Non-Mammalian Synapsids
Authors:
LeBlanc et al
Abstract:
Here we use the description of a new specimen of the small caseid synapsid Casea broilii that preserves the sacral, pelvic and hind limb regions in great detail and in three dimensions, as a unique opportunity to reevaluate the early stages in the evolution of the sacrum in the lineage that led to mammals. We place this new material in the context of sacral evolution in early caseid synapsids and conclude that the transition from two to three sacral vertebrae occurred in small-bodied species, suggesting that it was not an adaptation to heavy weight bearing. Furthermore, we compare descriptions of sacral anatomy among known early synapsids, including caseids, ophiacodontids, edaphosaurids, varanopids, and sphenacodontians and review sacral evolution in early synapsids. Based on the descriptions of new species of caseids, edaphosaurids, and varanopids over the past several decades, it is clear that a sacrum consisting of three vertebrae evolved independently at least four times in synapsids during the Late Carboniferous and Early Permian. Furthermore, similarities in the morphologies of the sacral vertebrae and ribs of these early synapsids lead us to conclude that an anterior caudal vertebra had been incorporated into the sacral series convergently in these groups. Given the repeated acquisition of a three-vertebra sacrum in early synapsids and no apparent link to body size, we argue that this sacral anatomy was related to more efficient terrestrial locomotion than to increased weight bearing.
Urodelans, Ichthyostega and the origin of the tetrapod limb
Author:
Mednikov
Abstract:
Available information on the development of primitive urodele (Hynobiidae) limbs and limb structure in Devonian tetrapods provide the basis for formulating the hypothesis on the existence of a special phase, the phase of the biserial limb in tetrapod history. The limb of the Devonian amphibian Ichthyostega with two groups of digits, preaxial and postaxial, corresponds well to this phase. Based on the structure of the Ichthyostega limb, it is suggested that the fin type ancestral to terrestrial limbs was asymmetrical biserial, possessed a short axis, unbranched jointed preaxial radials deviating one by one from each mesomere of the axis, and unbranched jointed postaxial radials deviating from the distal mesomeres of the axis in two or more radials.
Re-examination of a circa 100,000-year-old archaic early human skull found 35 years ago in Northern China has revealed the surprising presence of an inner-ear formation long thought to occur only in Neandertals.
"The discovery places into question a whole suite of scenarios of later Pleistocene human population dispersals and interconnections based on tracing isolated anatomical or genetic features in fragmentary fossils," said study co-author Erik Trinkaus, PhD, a physical anthropology professor at Washington University in St. Louis.
"It suggests, instead, that the later phases of human evolution were more of a labyrinth of biology and peoples than simple lines on maps would suggest."
The study, forthcoming in the Proceedings of the National Academy of Sciences, is based on recent micro-CT scans revealing the interior configuration of a temporal bone in a fossilized human skull found during 1970s excavations at the Xujiayao site in China's Nihewan Basin.
Trinkaus, the Mary Tileston Hemenway Professor in Arts & Sciences, is a leading authority on early human evolution and among the first to offer compelling evidence for interbreeding and gene transfer between Neandertals and modern human ancestors.
One of the quintessential depictions of prehistoric times is that of an ancient, often volcano ridden, landscape full of animals bearing large showy sails of skin stretched over their backs. Sailbacked animals are rather rare in our modern day and age, but back in the Mesozoic and Paleozoic there were sails a plenty.
link.
Neandertal clavicle length
Authors:
Trinkuas et al
Abstract:
The Late Pleistocene archaic humans from western Eurasia (the Neandertals) have been described for a century as exhibiting absolutely and relatively long clavicles. This aspect of their body proportions has been used to distinguish them from modern humans, invoked to account for other aspects of their anatomy and genetics, used in assessments of their phylogenetic polarities, and used as evidence for Late Pleistocene population relationships. However, it has been unclear whether the usual scaling of Neandertal clavicular lengths to their associated humeral lengths reflects long clavicles, short humeri, or both. Neandertal clavicle lengths, along with those of early modern humans and latitudinally diverse recent humans, were compared with both humeral lengths and estimated body masses (based on femoral head diameters). The Neandertal do have long clavicles relative their humeri, even though they fall within the ranges of variation of early and recent humans. However, when scaled to body masses, their humeral lengths are relatively short, and their clavicular lengths are indistinguishable from those of Late Pleistocene and recent modern humans. The few sufficiently complete Early Pleistocene Homo clavicles seem to have relative lengths also well within recent human variation. Therefore, appropriately scaled clavicular length seems to have varied little through the genus Homo, and it should not be used to account for other aspects of Neandertal biology or their phylogenetic status.
Motion range of the manus of Plateosaurus engelhardti von Meyer, 1837
Authors:
Reiss et al
Abstract:
The mode of locomotion of the basal sauropodomorph Plateosaurus engelhardti, known from numerous finds from the late Triassic of Central Europe, has been extensively debated. Some early and recent research results indicate that the forelimb could not play role in quadrupedal locomotion. Other authors suggested facultative or even permanent quadrupedality. This would require adaptations of the range of motion and the stability of the manual digits to the high forces caused by locomotion. An analysis of the hyperextension capabilities of the hand can therefore determine if the manus is adapted for locomotion. This study examines the capabilities of the manus of P. engelhardti using digital 3D modeling. The motion ranges of the digits were simulated in a computer-aided engineering (CAE) program, and the hyperextension capability of the entire manus was tested.
We find that the hand of Plateosaurus was not able to support the animal during quadrupedal locomotion, but may rather have been a specialized grasping organ. Therefore, P. engelhardti must have been an obligate biped.
Pedal Proportions of Poposaurus gracilis: Convergence and Divergence in the Feet of Archosaurs
Authors:
Farlow et al
Abstract:
The crocodile-line basal suchian Poposaurus gracilis had body proportions suggesting that it was an erect, bipedal form like many dinosaurs, prompting questions of whether its pedal proportions, and the shape of its footprint, would likewise “mimic” those of bipedal dinosaurs. We addressed these questions through a comparison of phalangeal, digital, and metatarsal proportions of Poposaurus with those of extinct and extant crocodile-line archosaurs, obligate or facultatively bipedal non-avian dinosaurs, and ground birds of several clades, as well as a comparison of the footprint reconstructed from the foot skeleton of Poposaurus with known early Mesozoic archosaurian ichnotaxa. Bivariate and multivariate analyses of phalangeal and digital dimensions showed numerous instances of convergence in pedal morphology among disparate archosaurian clades. Overall, the foot of Poposaurus is indeed more like that of bipedal dinosaurs than other archosaur groups, but is not exactly like the foot of any particular bipedal dinosaur clade. Poposaurus likely had a digitigrade stance, and its footprint shape could have resembled grallatorid ichnotaxa, unless digit I of the foot of Poposaurus commonly left an impression.
New research by a University of Rhode Island professor suggests that the length of human pregnancy is limited primarily by a mother's metabolism, not the size of the birth canal. The research, published in the Proceedings of the National Academy of Sciences the week of August 27, challenges the long-held notion of an evolutionary trade-off between childbirth and a pelvis adapted for walking upright.
Two traits that set humans apart from other primates—big brains and the ability to walk upright—could be at odds when it comes to childbirth. Big brains and the big heads that encase them are hard to push through the human birth canal, but a wider pelvis might compromise bipedal walking. Scientists have long posited that nature's solution to this problem, which is known as the "obstetric dilemma," was to shorten the duration of gestation so that babies are born before their heads get too big. As a result, human babies are relatively helpless and seemingly underdeveloped in terms of motor and cognitive ability compared to other primates.
"All these fascinating phenomena in human evolution—bipedalism, difficult childbirth, wide female hips, big brains, relatively helpless babies—have traditionally been tied together with the obstetric dilemma," said Holly Dunsworth, an anthropologist at the University of Rhode Island and lead author of the research. "It's been taught in anthropology courses for decades, but when I looked for hard evidence that it's actually true, I struck out."
The first problem with the theory is that there is no evidence that hips wide enough to deliver a more developed baby would be a detriment to walking, Dunsworth said. Anna Warrener, a post-doctoral researcher at Harvard University and one of the paper's co-authors, has studied how hip breadth affects locomotion with women on treadmills. She found that there is no correlation between wider hips and a diminished locomotor economy.
"That throws doubt on the assumption that the size of the birth canal is limited by bipedalism," Dunsworth said. "Wide hips don't mean you can't walk efficiently."
Then Dunsworth looked for evidence that human pregnancy is shortened compared to other primates and mammals. She found well-established research to the contrary. "Controlling for mother's body size, human gestation is a bit longer than expected compared to other primates, not shorter," she said. "And babies are a bit larger than expected, not smaller. Although babies behave like it, they're not born early."
For mammals in general, including humans, gestation length and offspring size are predicted by mother's body size. Because body size is a good proxy for an animal's metabolic rate and function, Dunsworth started to wonder if metabolism might offer a better explanation for the timing of human birth than the pelvis.
To investigate that possibility, she enlisted the help of Peter Ellison of Harvard University and Herman Pontzer of Hunter College in New York, two experts in human physiology and energetics. Building on Ellison's prior work on human pregnancy and childbirth, the researchers developed a new hypothesis for the timing of human birth called the EGG (energetics, gestation, and growth).
"Under the EGG, babies are born when they're born because mother cannot put any more energy into gestation and fetal growth," Dunsworth explains. "Mom's energy is the primary evolutionary constraint, not the hips."
Using metabolic data on pregnant women, the researchers show that women give birth just as they are about to cross into a metabolic danger zone.
