In the end, the research team recovered more than pieces of the skeleton, including much of the feet and virtually all of the hands—an extreme rarity among hominid fossils of any age, let alone one so very ancient. The team also found some 6, animal fossils and other specimens that offer a picture of the world Ardi inhabited: a moist woodland very different from the region's current, parched landscape. In addition to antelope and monkey species associated with forests, the deposits contained forest-dwelling birds and seeds from fig and palm trees.
Wear patterns and isotopes in the hominid teeth suggest a diet that included fruits, nuts, and other forest foods. If White and his team are right that Ardi walked upright as well as climbed trees, the environmental evidence would seem to strike the death knell for the "savanna hypothesis"—a long-standing notion that our ancestors first stood up in response to their move onto an open grassland environment. One provocative answer to that question—originally proposed by Lovejoy in the early s and refined now in light of the Ardipithecus discoveries—attributes the origin of bipedality to another trademark of humankind: monogamous sex.
Virtually all apes and monkeys, especially males, have long upper canine teeth—formidable weapons in fights for mating opportunities. But Ardipithecus appears to have already embarked on a uniquely human evolutionary path, with canines reduced in size and dramatically "feminized" to a stubby, diamond shape, according to the researchers. Males and female specimens are also close to each other in body size.
Lovejoy sees these changes as part of an epochal shift in social behavior: Instead of fighting for access to females, a male Ardipithecus would supply a "targeted female" and her offspring with gathered foods and gain her sexual loyalty in return.
To keep up his end of the deal, a male needed to have his hands free to carry home the food. Bipedalism may have been a poor way for Ardipithecus to get around, but through its contribution to the "sex for food" contract, it would have been an excellent way to bear more offspring.
And in evolution, of course, more offspring is the name of the game. Two hundred thousand years after Ardipithecus, another species called Australopithecus anamensis appeared in the region. By most accounts, that species soon evolved into Australopithecus afarensis, with a slightly larger brain and a full commitment to a bipedal way of life.
Then came early Homo, with its even bigger brain and budding tool use. Did primitive Ardipithecus undergo some accelerated change in the , years between it and Australopithecus —and emerge as the ancestor of all later hominids? Or was Ardipithecus a relict species, carrying its quaint mosaic of primitive and advanced traits with it into extinction?
Study co-leader White sees nothing about the skeleton "that would exclude it from ancestral status. Stony Brook's Jungers added, "These finds are incredibly important, and given the state of preservation of the bones, what they did was nothing short of heroic. All rights reserved. Move over, Lucy. However, there is no special relationship between plantigrade foot postures and knuckle-walking hand postures e. The hand of Ar. The terrestrial specializations of the hominine foot are likely to be homologous because they are present in Ar.
Critically, they carry a similar set of implications for the origin of bipedalism regardless of the terrestrial hand posture of the Homo-Pan LCA. This study provides evidence that modern humans evolved from an ancestor with an African ape-like foot associated with terrestrial plantigrady and vertical climbing. Hominin upright walking therefore likely emerged in the context of semi-terrestrial quadrupedalism.
Explaining the adaptive origin of hominin bipedalism i. However, the comparative and fossil material provides evidence for patterns of evolution i. The Ar. This study provides evidence that intrinsic foot proportions reflect locomotor diversity among anthropoid primates, but it will be important to consider other regions in future comparative studies.
The hypothesis that hominins evolved from a semi-terrestrial quadrupedal ancestor could be tested with detailed quantitative analyses of other aspects of Ar.
The extant sample is composed individuals representing 45 taxa: Homo sapiens , Ardipithecus ramidus , Pan troglodytes , Pan paniscus , Gorilla beringei beringei , Gorilla beringei graueri , Gorilla gorilla , Pongo pygmaeus , Pongo abelii , Hylobates lar , Hylobates muelleri, Hylobates klossii, Hylobates agilis, Hoolock hoolock, Symphalangus syndactylus , Macaca fascicularis , Macaca nemestrina , Papio anubis, Papio hamadryas, Theropithecus gelada , Lophocebus albigena , Mandrillus sphinx , Mandrillus leucophaeus, Cercocebus spp.
Six measurements were taken on the foot of each individual using Mitutoyo digital calipers: maximum talar articular length, talar trochlea length, cuboid length, first metatarsal MT1 length, fifth metatarsal length MT5 , and fourth proximal phalanx PP4 length. Talar neck length was derived by subtracting the talar trochlea length from maximum talar articular length.
Maximum talar articular length is defined as the maximum proximodistal distance between the most proximal margin of the talar trochlea and the most distal point on the talar head. Talar trochlea length is defined as the maximum proximodistal distance between the most proximal margin of the talar trochlea and the most distal point of the talar trochlea. Cuboid length is defined as the proximodistal distance between the dorsal margin of the calcaneal facet and the most distal point of the tarsometatarsal joint, taken in dorsal view in approximate anatomical orientation.
The purpose of measuring cuboid length in this manner is to explicitly avoid the cuboid beak or calcaneal process since it varies extensively among great apes Lewis, and because it confounds the cuboid length measurement as a representation of midfoot length since it is articular and housed within a corresponding concavity on the cuboid facet of the calcaneus.
MT1 length is defined as the maximum proximodistal distance between the most proximal points on the metatarsal base with calipers held flush and the most distal point of the metatarsal head. MT5 length is defined as the proximodistal distance between the most proximal point of the cuboid-MT4 articular margin and the most distal point on the metatarsal head. PP4 length is defined as maximum proximodistal distance between the most proximal point of the phalangeal base and the most distal point of the trochlea.
The fossil is missing most of its metatarsal head and its length was estimated by Lovejoy et al. There is a nearly complete third metatarsal of Ar. There is a partially preserved second metatarsal of Ar. The individual elements of the bony foot skeleton contribute to the production of three movements used in various forms of primate locomotor behavior that are hypothesized to be reflected in intrinsic foot proportions: hallucal adduction and flexion, non-hallucal digital flexion, and plantarflexion at the talocrural joint.
Increasing the length of the first metatarsal should increase the moment arm of the intrinsic adductor musculature such as the m. Increasing hallucal metatarsal and non-hallucal phalangeal lengths also contributes to increasing the span of the pedal grasp in taxa with a mobile hallux, which also helps to maintain a friction grip in pedal grasping Cartmill, Previous studies have modeled the foot skeleton as a second-class lever, where the fulcrum is at the metatarsophalangeal joints, the load passes through the talocrural joint at the rearfoot, and the force is produced by the plantarflexor muscles.
Increasing the effort arm of the foot relative to the load arm increases the mechanical advantage of the foot skeleton as a lever Schultz, a ; Schultz, b ; Strasser, However, increasing the load arm increases the range of motion for a given amount of plantarflexor contraction Schultz, a ; Schultz, b.
The length of the effort arm of the Ar. The length of the load arm can be increased by lengthening the metatarsals, tarsals e. Increasing the length of the metatarsals subjects them to greater bending moments during stance phase, and therefore increases the possibility of injury, so humans achieve a longer load arm by instead increasing the length of the tarsals.
As such, in a bipedal heel-strike plantigrade foot, the load arm is increased by lengthening the cuboid and other midtarsal elements. In contrast, in quadrupedal semiplantigrade or digitigrade primates, and indeed other terrestrial cursorial mammals Taylor, , the load arm is lengthened by increasing the length of the metatarsals. One implication of these differing anatomical arrangements e. To correct for differences in scale among species, each measurement was divided by the geometric mean of all six measurements per individual Jungers et al.
Morphometric affinities were evaluated using an unweighted pair-group with arithmetic mean UPGMA cluster analysis on Euclidean distances. The cophenetic correlation coefficient was used to assess the degree to which the resulting dendrogram represented the true pairwise distances between taxa Sokal and Rohlf, Principal Components Analysis PCA on all six geometric mean-standardized variables was used to reduce, ordinate, and visualize the multivariate data.
All evolutionary modeling and ancestral state estimation analyses use the first three principal components PCs derived from the PCA. The PC scores were used instead of the original data to avoid analytical problems surrounding correlations among variables Clavel et al.
Ardipithecus ramidus was added to a molecular phylogenetic tree from 10 k trees Arnold et al. The branch length for Homo sapiens was reduced in order to improve estimation of phenotypic optima in evolutionary analyses Butler and King, ; Ingram and Mahler, A phylomorphospace was constructed by superimposing the phylogenetic tree on the average principal component PC scores for each taxon and ancestral values were estimated using a Markov chain Monte Carlo method MCMC that relaxes assumptions of neutrality and gradualism Elliot and Mooers, Ancestral values estimated under constant rate Brownian motion are affected by taxa that are exceptionally phenotypically derived compared to their close relatives because it is assumed that all branches have evolved at the same rate Elliot and Mooers, Therefore, estimates of ancestral values for continuous traits that assume a constant evolutionary rate are potentially biased in the direction of more derived branches characterized by higher phenotypic evolutionary rates.
Since this study is focused on estimating ancestral values for humans, great apes, and hominoids, the stable model Elliot and Mooers, was specifically chosen in light of evidence for molecular and morphological evolutionary rate differences in hominoids relative to other anthropoids Steiper et al.
Ancestral states PC scores were estimated under a constant rate Brownian motion model and a stable model using StableTraits software version 1. Two independent Markov chains were run with 2,, iterations at a thinning rate of , resulting in 10, samples each. Priors on evolutionary rate were set to the default settings as implemented in StableTraits and which prevent rates from approaching zero.
The two chains converged after , iterations as evidenced by a proportional scale reduction factor PSRF value approaching 1 Brooks and Gelman, Therefore, the first , iterations for each of the two chains were discarded as burn in. A model-based approach was used to evaluate alternative evolutionary hypotheses: Brownian motion, single-optimum Ornstein-Uhlenbeck OU , and multi-optimum OU. Under Brownian motion all trait changes are independent of previous ones, as well as those on other branches, and trait variance is proportional to time i.
As such, individual species may differ significantly from one another while simultaneously occupying the same global phenotypic optimum, possibly due to other factors such as drift or pleiotropy Hansen, The evolutionary hypotheses for the link between foot proportions and behavior are informed by observations of locomotor behavior in the wild reported in the literature. The first evolutionary hypothesis is a Brownian motion model H 1.
The second evolutionary hypothesis is the first Hansen model and it reflects a single global phenotypic optimum H 2. Support for these hypotheses would suggest that there are no major adaptive differences in foot proportions between anthropoid primate groups. Subsequent hypotheses represent multi-optima OU models of increasing complexity i. The second Hansen model has three selective regimes associated with advanced bipedalism Homo , mostly terrestrial quadrupedal locomotion Pan , Gorilla , Papio , Theropithecus , Erythrocebus, Chlorocebus aethiops and mostly arboreal quadrupedal locomotion in all other taxa H 3.
African apes are competent climbers and possess foot adaptations related to this form of locomotion DeSilva, However, observations from the wild show them to be highly terrestrial.
Papio , Theropithecus , Erythrocebus, and Chlorocebus aethiops are the most terrestrial among the cercopithecoids. Previous studies suggest there may be effects of arboreality and terrestriality on foot proportions Schultz, a ; Jolly, The third Hansen model is an elaboration of the previous one that splits the terrestrial regime into two: terrestrial heel-strike plantigrady in Pan , and Gorilla , and terrestrial semiplantigrady in Papio , Theropithecus , Erythrocebus, and Chlorocebus aethiops H 4.
Semiplantigrady is defined as any habitual foot posture in which some, but not all, tarsals are in contact with the substrate during stance phase.
Heel-strike plantigrady is defined as a foot posture in which the tarsals, principally the calcaneus and its proximal tuberosity, contact the substrate at the beginning of stance phase. Several studies have suggested that African ape tarsal morphology reflects their heel-strike plantigrade foot posture Gebo, ; Gebo, Previous work on mammalian foot proportions implies that foot proportions may reflect foot posture Taylor, The Australian Museum respects and acknowledges the Gadigal people as the First Peoples and Traditional Custodians of the land and waterways on which the Museum stands.
Image credit: gadigal yilimung shield made by Uncle Charles Chicka Madden. This website uses cookies to ensure you get the best experience on our website. Learn more. Background of discovery Age 4. It consists of teeth and jaw bone and was found in Aramis in It is the oldest known skeleton of a human ancestor.
She weighed about 50kg and stood about cm tall. The skeleton was in extremely poor condition and it took the team 15 years to excavate, scan, make virtual reconstructions, assemble and then analyse.
The results were hugely significant in terms of how we view the evolution of the earliest hominins and the physical appearance of the last common ancestor of humans and chimpanzees.
The skeleton does not look much like a chimp or gorilla or have the expected 'transitional' features. Instead, it may well preserve some of the characteristics of the last chimp-human ancestor. Analysis of the skeleton reveals that humans did not evolve from knuckle-walking apes, as was long believed. It also indicates that chimpanzee evolution underwent high degrees of specialisation since diverging from the last common ancestor and thus these apes are poor models for understanding the appearance of this ancestor.
Below are some of the still unanswered questions about Ardipithecus ramidus that may be answered with future discoveries : Does the pelvis of Ar. The pelvis was reconstructed from crushed fossils and, according to some scientists, is only suggestive of bipedalism. What is the average size of male Ar.
If more fossils support the original finding of relatively low sexual dimorphism, how does this relate to male and female size differences in other early humans at the base of our family tree -- and what does it mean?
References: First paper: White, T. Other recommended readings: Gibbons, A. A new kind of ancestor: Ardipithecus unveiled. Chickens, chimpanzees, and you - what do they have in common?
Grandparents are unique to humans How strong are we? Humans are handy! Humans: the running ape Our big hungry brain! Our eyes say it! The early human tool kit The short-haired human!
0コメント