A characteristic of reports about human ancestors has been the hailing of a particular fossil species as a critical link in the evolutionary chain. Invariably, this gets plenty of media publicity. With the passing of time, the discoveries do not seem so dramatic and the puzzles and challenges appear unresolved. We are currently witnessing this pattern of discourse in relation to Australopithecus sediba, known from two specimens recovered in 2008 from the Malapa Cave site in South Africa and published in 2010. The most recent analyses appeared in Science in April 2013, and some have suggested "that A. sediba may just be the most important hominin (modern humans and their extinct relatives) discovery yet." The New Scientist report commenced thus:
"Our closest non-human ancestor lived in South Africa. That's the conclusion of a battery of studies carried out on two strange skeletons discovered near Johannesburg in 2008. They represent a likely stepping stone between the ape-like australopiths and the first members of our own genus."
Fossil elements of the two partial skeletons of Australopithecus sediba. The grey shadows represent skeletons of A. africanus, adjusted for different body proportions. (Credit: Science/AAAS. Source here)
It is customary for the discoverer and co-workers to present the strongest arguments they can find to attract interest in their research. In this case, the thesis is that A. sediba is a "mosaic of ancient australopith and modern Homo features" and that this makes it convincing as a transitional form. An independent assessment of the findings has been made by William Kimbel in Nature. He is not convinced.
"[The two skeletons] have been the focus of scrutiny because of both their excellent preservation and claims that this hominin - a species more closely related to humans than to chimpanzees - lies at the base of the Homo lineage. A series of reports published in Science sheds light on the morphology of A. sediba but, in my view, does little to elucidate its role in later human evolution." (p.573)
First, Kimbel looks at dental morphology. He finds methodology problems that raise serious questions about the conclusions drawn.
"The researchers take] the unconventional step of using only the Arizona State University Dental Anthropology System - a graded series of minor crown variants originally devised to distinguish recent human populations from one another - to decipher relationships between hominin species that are millions of years old. I have serious doubts about the phylogenetic meaning of morphological similarity in this case. These concerns are compounded by the authors' reliance on the gorilla as the sole outgroup in their cladistic analysis." (p.573)
Second, analysis of the mandible suggests that insufficient attention has been given to the samples comprising a sub-adult (with only its second molar erupted) and a presumed adult female.
Third, the upper and lower limb dimensions show many ape-like affinities and suggest arboreal climbing behavior. The New Scientist report refers to this as an "unusual feature of A. sediba: its arms and legs show it was far more comfortable swinging in the trees than most australopiths."
Fourth, the rib cage and vertebral column was claimed to be Homo-like. Kimbel raises questions about what is primitive and what derived and suggests that there is nothing in the fossil material that can be regarded as unexpected.
Finally, the issue of gait is considered. Ann Gibbons drew attention to sediba's unusual way of walking in an article for Science:
"If you happened to be in South Africa about 2 million years ago, you might have seen an odd sight: an older female hominin sashaying down a wooded slope, perhaps in search of water. She walked upright, but she wasn't human, and she moved with what to our eyes would have looked like a distinctly strange gait. She was a member of Australopithecus sediba, and [. . .] she may have twisted from side to side, rolling her feet inward with each step. "Sediba's got swag," says paleoanthropologist Lee Berger of the University of the Witwatersrand in Johannesburg, South Africa. (p.132)
Kimbel is not impressed. He describes the research conclusions as controversial.
"The proposed 'hyperpronation' of the foot and extreme inward rotation of the leg and thigh suggest an ungainly bipedal stride that might have made it into Monty Python's 'Ministry of Silly Walks' sketch. The presumed inversion of the foot at the heel-strike of the surprisingly ape-like calcaneus, combined with a vertical shank (tibia), outwardly angled thigh (femur) and a long, lordotic lower back - all hallmarks of terrestrial bipedality in Australopithecus and Homo species - constrains the reconstruction. Prominent osteophytic growths on the pelvis and fibula at the attachment sites of the thigh musculature raise the possibility of a gait that was pathologically impaired, but DeSilva and colleagues argue that this locomotor pattern was adaptive. However, if A. sediba was a descendant of A. africanus, which, similarly to the even older A. afarensis (dating to between 3.7 million and 3.0 million years ago), shows no trace of this pattern, then it is hard to imagine the selective advantage that would accrue from such a kinematically peculiar gait." (p.574)
The general conclusion from Kimbel is that the grounds for claiming A. sediba to be a transitional fossil in the human lineage are weak. The research tells us more about the Australopithecenes, but little more.
"Given the mix of features seen in A. sediba, it is difficult to understand why these researchers insist that it lies at the base of the Homo lineage. Similar intellectual gymnastics are required to comprehend the authors' argument that no African Homo fossils exist from before the time of A. sediba. Although the recent papers constitute a fascinating further analysis of the A. sediba fossils, I do not think that they provide compelling evidence that this species is anything other than an unusual australopith from a Pliocene-Pleistocene time period that is already populated by a fair number of them." (p.574)
Hesitation on hominin history
William H. Kimbel
Nature, 497, 573-574 (30 May 2013) | doi:10.1038/497573a
Extensive studies of fossil skeletons of Australopithecus sediba provide fascinating details of the anatomy of this hominin species, but do not convincingly indicate its position on the evolutionary route to modern humans.
Insect eyes may be tiny but they belong to a "remarkably sophisticated class of imaging systems". They have compound eyes, made up of hundreds or thousands of optical units or facets. Considered as optical systems, these facets are simpler than the vertebrate eye design because there are no moving parts to focus the light.
"In the case of the 'apposition' eye of daylight insects, each facet is optically isolated from its neighbour and equipped with its own lens and set of photoreceptors. Because each facet accepts photons from only a small angle in space, the light sensitivity of apposition eyes is rather low and the spatial resolution is limited by the number of facets that can be packed on to the small head of the insect. However, apposition eyes provide their bearer with a panoramic view of the world as well as with an infinite depth of field, without the need to adjust the focal length of the individual lenses." (Borst & Plett, 2013, 47)
The hemispherical digital camera with nearly 200 tiny lenses (Credit: John A. Rogers, University of Illinois at Urbana-Champaign, Source here)
In addition to the wide field of view and the infinite depth of field, compound eyes have low aberrations and high acuity to motion. This latter characteristic is very desirable for flying insects and for micro aerial vehicles (MAVs). Thus far, most MAVs have been fitted with fisheye lenses that provide a wide-angle field of view. However, the information gathered has limitations for calculating the special movement that is required to facilitate motion stabilisation alongside navigation.
"With regard to potential applications, the camera proposed by Song et al. might constitute an optimal front-end visual sensor for tiny aircraft called micro aerial vehicles (MAVs). [. . .] Song and colleagues' camera would provide all the advantages of an apposition eye. Using it to compute a MAV's self-motion could on the one hand facilitate motion stabilization in space while on the other enabling spatial navigation." (Borst & Plett, 2013, 48)
Several research groups have taken up the challenge of developing "compound" cameras that mimic a hemispherical arthropod eye. Precision engineering with dimensions measured in micrometres must be combined with recently devised electronic technologies to achieve the goal. The research paper describes the use of elastomeric compound optical elements, deformable arrays of thin-film silicon photodetectors, and the means by which the necessary alignments can be achieved.
"Here we present a complete set of materials, design layouts and integration schemes for digital cameras that mimic hemispherical apposition compound eyes found in biology. Certain of the concepts extend recent advances in stretchable electronics and hemispherical photodetector arrays, in overall strategies that provide previously unachievable options in design. Systematic experimental and theoretical studies of the mechanical and optical properties of working devices reveal the essential aspects of fabrication and operation." (Song et al., 2013, 95).
By adopting the strategy of biomimicry, the distinctive characteristics of compound eyes are clarified.
"The arthropod eye offers resolution determined by the numbers of ommatidia, and is typically modest compared, for example, to mammalian eyes. Two other attributes, however, provide powerful modes of perception. First, the hemispherical apposition design enables exceptionally wide-angle fields of view, without off-axis aberrations. [. . .] The second attribute is the nearly infinite depth of field that results from the short focal length of each microlens and the nature of image formation. In particular, as an object moves away from the camera, the size of the image decreases but remains in focus. A consequence is that the camera can accurately and simultaneously render pictures of multiple objects in a field of view, even at widely different angular positions and distances. [. . .] Even though movement of the object away from the camera changes its size in the corresponding image, the focus is maintained. Objects with the same angular size that are located at different distances produce images with the same size, all of which is consistent with modelling." (Song et al., 2013, 97-98).
We should note that arthropod eye designs do not conform to the cobbled-together architecture that is associated with evolutionary tinkering. The driver for biomimetics research is the recognition of exquisite design that extends and inspires human creativity. Exquisite design is the hallmark of an intelligent agent. For more on this point, go here.
Recent discoveries of fossilised compound eyes of the Middle Cambrian predator Anomalocaris, and of an Early Cambrian arthropod, have documented clear examples of modernity in some of the earliest known arthropods. These exquisite fossil eyes provide evidence of abrupt appearance in the fossil record. There is no 'audit trail' here to support blind, tinkering, evolutionary transformation. Thus, biomimetic research and the fossil record provide complementary sources of evidence for intelligent design.
Digital cameras with designs inspired by the arthropod eye
Young Min Song, Yizhu Xie, Viktor Malyarchuk, Jianliang Xiao, Inhwa Jung, Ki-Joong Choi, Zhuangjian Liu, Hyunsung Park, Chaofeng Lu, Rak-Hwan Kim, Rui Li, Kenneth B. Crozier, Yonggang Huang & John A. Rogers
Nature, 497, 95-99 (02 May 2013) | doi:10.1038/nature12083
In arthropods, evolution has created a remarkably sophisticated class of imaging systems, with a wide-angle field of view, low aberrations, high acuity to motion and an infinite depth of field. A challenge in building digital cameras with the hemispherical, compound apposition layouts of arthropod eyes is that essential design requirements cannot be met with existing planar sensor technologies or conventional optics. Here we present materials, mechanics and integration schemes that afford scalable pathways to working, arthropod-inspired cameras with nearly full hemispherical shapes (about 160 degrees). Their surfaces are densely populated by imaging elements (artificial ommatidia), which are comparable in number (180) to those of the eyes of fire ants (Solenopsis fugax) and bark beetles (Hylastes nigrinus). The devices combine elastomeric compound optical elements with deformable arrays of thin silicon photodetectors into integrated sheets that can be elastically transformed from the planar geometries in which they are fabricated to hemispherical shapes for integration into apposition cameras. Our imaging results and quantitative ray-tracing-based simulations illustrate key features of operation. These general strategies seem to be applicable to other compound eye devices, such as those inspired by moths and lacewings (refracting superposition eyes), lobster and shrimp (reflecting superposition eyes), and houseflies (neural superposition eyes).
Borst, A. & Plett, J., Seeing the world through an insect's eyes, Nature, 497, 47-48 (02 May 2013) | doi:10.1038/497047a
Beciri, D., Arthropod eye design inspires novel digital cameras, rob-aid, (2 May 2013)
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