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This book sets out to reconstruct the evolutionary history of the human face, in terms of both the fossil evidence and the recent findings of genetics, molecular biology, and developmental biology that have illuminated how the human face forms during embryonic and fetal development. In exploring this history, we will see how intimately the evolution of the face was connected to that of the brain and how mental and social processes have helped shape the human face; intriguingly, those processes have continued well into the recent history of our species. Along the way, we will take note of the remarkable diversity of human faces and examine the genetic foundations of that diversity, findings relevant to understanding the (probable) evolutionary future of the face. The final chapter sums up the key features of the history of the face, and explores how that history illuminates human evolution specifically and exemplifies the evolutionary process in general.-- Provided by publisher

Infants across species are thought to exhibit specific facial features (termed the “baby schema”, such as a relatively bigger forehead and eyes, and protruding cheeks), with an adaptive function to induce caretaking behaviour from adults. There is abundant empirical evidence for this in humans, but, surprisingly, the existence of a baby schema in non-human animals has not been scientifically demonstrated. We investigated which facial characteristics are shared across infants in five species of great apes: humans, chimpanzees, bonobos, mountain gorillas, and Bornean orangutans. We analysed eight adult and infant faces for each species (80 images in total) using geometric morphometric analysis and machine learning. We found two principal components characterizing infant faces consistently observed across species. These included (1) relatively bigger eyes located lower in the face, (2) a rounder and vertically shorter face shape, and (3) an inverted triangular face shape. While these features are shared, human infant faces are unique in that the second characteristic (round face shape) is more pronounced, whereas the third (inverted triangular face shape) is less pronounced than other species. We also found some infantile features only found in some species. We discuss future directions to investigate the baby schema using an evolutionary approach.

An outline for an organismic theory of reproductive tactics is presented to develop the demographic theory of optimal reproductive tactics into a more realistic theory of life-history evolution. Reproductive effort-defined as the proportion of resources invested in reproduction-and the costs in somatic investment do not automatically result in survival costs. Both the conditions where survival costs are produced and the conditions where reproduction can take place without survival costs are specified. Compensation and threshold hypotheses are put forward to allow weaker correlations between reproduction and survival than the trade-off hypothesis, which assumes direct impacts by reproductive effort on survival. Furthermore, reproductive tactics are unlikely to be moulded by the demographic forces of selection only. An empirical example is shown where residual reproductive value played no significant role in the evolution of reproductive tactics. Selection probably operates not on separate life-history traits but on whole organisms through their entire life-history. The structural and physiological intercouplings between separate traits can result in phenotypic opportunity sets where selection can mould life-history traits only within the constraints of the opportunity sets. Optimization theory has provided an efficient technique for modelling and making predictions. However, organismic selection does not necessarily optimize adaptive strategies but eliminates unfit strategies. Life-history theory, and evolutionary theory in general, can be developed along alternative logical lines when different hypotheses are generated on how selection operates.

The geographic variation in the length of the larval period and the size at metamorphosis of the wood frog, Rana sylvatica, is examined for populations in the tundra of Canada, the mountains of Virginia, and the lowlands of Maryland. We argue that the observed differences in developmental plasticity, heritabilities and genetic covariances of traits among localities result from differential selection pressures in each environment, and are related to the physiological constraints inherent in development and to the degree of compromise between the timing and size at metamorphosis allowed in each environment. In Maryland populations fitness has been maximized by evolutionary changes in size alone; body size in this population is canalized, has low heritability and is highly correlated with juvenile survival relative to developmental time. In Canada, minimum developmental time yields maximum fitness; the length of the larval period in this population is canalized and genetically monomorphic relative to body size. In contrast, fitness in the Virginia populations has been determined by correlated and pleiotropic effects of genes on both developmental time and larval body size, and both traits are equally canalized, affect juvenile survivorship equally and display moderate heritabilities. These results stress the importance of interpreting variation in life-history traits relative to constraints inherent in development and those imposed by the environment. Heritability and survivorship data support the general notion that fitness traits should have low levels of additive genetic variation, but also suggest that antagonistic pleiotropy may act to preserve genetic variation in fitness traits under simultaneous selection, and caution against inferring evolutionary importance of individual traits without considering the possible presence of pleiotropy.

The developmental mechanisms by which the environment may alter the phenotype during development are reviewed. Developmental plasticity may be of two forms: developmental conversion or phenotypic modulation. In developmental conversion, organisms use specific environmental cues to activate alternative genetic programs controlling development. These alternative programs may either lead to alternative morphs, or may lead to the decision to activate a developmental arrest. In phenotypic modulation, nonspecific phenotypic variation results from environmental influences on rates or degrees of expression of the developmental program, but the genetic programs controlling development are not altered. Modulation, which is not necessarily adaptive, is probably the common form of environmentally induced phenotypic variation in higher organisms, and adaptiveness of phenotypic plasticity therefore cannot be assumed unless specific genetic mechanisms can be demonstrated. The genetic mechanisms by which developmental plasticity may evolve are reviewed, and the relationship between developmental plasticity and evolutionary plasticity are examined.

Although much life-history theory assumes otherwise, most life-history traits exhibit phenotypic plasticity in response to environmental factors during development. Plasticity has long been recognized as a potentially important factor in evolution, is known to be under genetic control, and may or may not be adaptive. The notion of adaptive plasticity contrasts with the idea that developmental homeostasis is a major evolutionary goal. The conflict was resolved in principle by Ashby's cybernetic analysis of homeostasis, which showed how plasticity in \"response variables\" might act to screen \"essential variables\" from the impact of environmental disturbance. To apply this analysis to life-history plasticity, it must be incorporated into a demographic model. An approach is presented here using life cycle graphs and matrix projection models. Plasticity in response to temporal variation leads to time-varying matrix models: plasticity in response to spatial variation leads to models structured by criteria other than age. The adaptive value of such plasticity can be assessed by calculating its effects on a suitable measure of fitness: long-term growth rate for time-invariant models, expected growth rate discounted by variance for time-varying models. Three examples are analyzed here: plasticity in the rate of development from one instar to the next in a stage-classified model, plasticity in multiplicative yield components, and plasticity in dormancy as a response to environmental cues. Development rate plasticity is adaptive if reproductive value increases from the instar in question to the next, maladaptive otherwise. Plasticity in yield components reduces fitness variance, and hence is adaptive, if the responses of successive developmental steps (e.g., flowers/stem, seeds/flower) are negatively correlated. Plasticity in dormancy is adaptive if it responds to the same factor(s) influencing mortality, but with opposite sign. A number of important problems, including trade-offs between genetic and phenotypic adaptation and the distinction between continuous and discontinous plasticity remain to be solved.

Two subspecies of the tiger salamander, Ambystoma tigrinum, have a distinctive polymorphic life history that can include four adult morphs as well as typical and cannibalistic larval morphs. We evaluated the effect of environment on development of larval morphology in two laboratory experiments. In Experiment I, 180 larvae were raised in individual 3-liter containers and fed one of three food levels. Larvae in Experiment II received one of two levels of food, and were raised at three densities: one larva per 3 liters of water (50 containers), three larvae per 22 liters of water (18 containers), or seven larvae per 22 liters of water (18 containers). Cannibalistic morphs developed only in nine containers at the highest density, and their occurrence was independent of the two food levels. Our results suggest the typical and cannibalistic larvae which occur in some populations of Ambystoma tigrinum nebulosum is an environmentally induced developmental polymorphism that results from some individuals responding to the environment differently than others. This difference in response may or may not be associated with genetic differences between these morphs. Based on our results we cannot discriminate between two models that differ in their assumptions about the genetic background of individual larvae.

In 1905, mosquitofish were introduced to sugar plantation reservoirs in Hawaii. Collections of at least 250 fish from each of 24 reservoirs, 4 stable and 20 fluctuating in water level, demonstrated that there were small but significant differences in the life history traits of fish from stable and fluctuating classes of reservoirs, and large and significant differences among stocks from individual reservoirs. Fish from 2 stable and 4 fluctuating reservoirs were then raised in individual containers with controlled food and temperature. Age and length at maturity, growth rates, and size of offspring all differed significantly among stocks. Broad-sense heritabilities were significantly greater than zero for female age at maturity for fish from one of two stable reservoirs, and for male maturation traits for fish from two of four fluctuating reservoirs. Rates of evolution, calculated from the maximum difference between the means of lab-raised stocks and assuming 140 generations since 1905 and continuous change, ranged from 0.1% to 0.5% of the average value of the trait per generation. The traits that changed more rapidly were also more phenotypically plastic, thus suggesting that phenotypic plasticity cannot account for stasis in the fossil record. The concept of plastic trajectories is introduced and exemplified, and predictions are made about how age and length at maturity should alter under stress for organisms with different demographic histories.

Prebiotic synthesis of short length macromolecules from precursor molecules results in a dynamic of spontaneous creation, which allows for growth from zero density. At this prereplicator stage in the evolution of life there is no life history, since the birth and death processes are intimately coupled through the physical chemistry of a single reaction. With the emergence of nonenzymatic, template-directed replication, the birth and death processes could diverge for the first time, since selection could act differently on the birth and death rates of the replicating molecule. Thus, with replication, natural selection and life-history evolution began. The genotype, or nucleotide sequence, of the replicating molecule gave rise to several phenotypic properties, the most important of which was its three-dimensional structure which in turn affected the birth and death processes. However, at this stage of nonenzymatic template replication, the phenotype was the physical structure of the genotype, nothing more. For the divergence of the phenotype from the genotype it was necessary for the replicator to produce a protein. It is shown here that the evolution of enzyme production is facilitated by the existence of population structure in the distribution of the macromolecules associated with replication. Initially, this structure was created passively by the localization of the macromolecules in rock crevices, suspended water droplets, etc. Ultimately, the replicator along with its proteins were localized in a protocellular structure and this became the first organism. Thus, initially, the organism was one extreme of the population structure of the macromolecules associated with life. The organism was the culmination of the encapsulation phase of evolution which proceeded through initial phases of passive localization.

New human fossils from Jebel Irhoud (Morocco) document the earliest evolutionary stage of Homo sapiens and display modern conditions of the face and mandible combined with more primative features of the neurocranium. Early dawn for Homo sapiens The exact place and time that our species emerged remains obscure because the fossil record is limited and the chronological age of many key specimens remains uncertain. Previous fossil evidence has placed the emergence of modern human biology in eastern Africa around 200,000 years ago. In this issue of Nature , Jean-Jaques Hublin and colleagues report new human fossils from Jebel Irhoud, Morocco; their work is accompanied by a separate report on the dating of the fossils by Shannon McPherron and colleagues. Together they report remains dating back 300,000–350,000 years. They identify numerous features, including a facial, mandibular and dental morphology, that align the material with early or recent modern humans. They also identified more primitive neurocranial and endocranial morphology. Collectively, the researchers believe that this mosaic of features displayed by the Jebel Irhoud hominins assigns them to the earliest evolutionary phase of Homo sapiens . Both papers suggest that the evolutionary processes behind the emergence of modern humans were not confined to sub-Saharan Africa. Fossil evidence points to an African origin of Homo sapiens from a group called either H. heidelbergensis or H. rhodesiensis . However, the exact place and time of emergence of H. sapiens remain obscure because the fossil record is scarce and the chronological age of many key specimens remains uncertain. In particular, it is unclear whether the present day ‘modern’ morphology rapidly emerged approximately 200 thousand years ago (ka) among earlier representatives of H. sapiens 1 or evolved gradually over the last 400 thousand years 2 . Here we report newly discovered human fossils from Jebel Irhoud, Morocco, and interpret the affinities of the hominins from this site with other archaic and recent human groups. We identified a mosaic of features including facial, mandibular and dental morphology that aligns the Jebel Irhoud material with early or recent anatomically modern humans and more primitive neurocranial and endocranial morphology. In combination with an age of 315 ± 34 thousand years (as determined by thermoluminescence dating) 3 , this evidence makes Jebel Irhoud the oldest and richest African Middle Stone Age hominin site that documents early stages of the H. sapiens clade in which key features of modern morphology were established. Furthermore, it shows that the evolutionary processes behind the emergence of H. sapiens involved the whole African continent.