The hierarchy of persuasive writing on scientific matters has
grades and inter-grades, but we can recognise the following -
1. Opinion, uninformed. Ideas that are full of bias, uninformed by fact, pungent with prejudice.
2. Opinion, misinformed. Ideas that are taken from unreliable sources such as magazines, popular books, and propagandists.
3. Opinion, informed. Ideas using current hypotheses and selected facts to draw and defend an entrenched position.
4. Web Pseudopaper. Speculative ideas (usually, rather than data) drawn from various scholarly authors, leaning heavily on obscure references and abstracts as well as secondary or tertiary sources, particularly the internet. Usually without any form of scholarly review, errors of fact often uncorrected, prone to excessive verbosity, often poorly structured, and sometimes with conclusions unsupported by the evidence cited. May be written by someone with little formal instruction in scientific methodology, or someone instructed in scientific methodology, but writing outside their field of study. The writing can only gain an audience because the internet exists. The paper may or may not be updated or changed as errors of fact and interpretation are drawn to the authors attention. (This last element, the possibility of continuous revision, is a distinctive element of a pseudopaper that is never present in a scholarly journal paper.)
5. Web Pseudopaper, informed. Usually presented informally by a scholarly author, ideas drawn from one or various authors, with or without unpublished data, intended to persuade or enlighten without the constraint of formal academic language and peer review, often designed to lead an argument in the authors favored direction, or present informal results. May be unedited for grammar, style and readability, relying on the clearly scholarly foundation to outweigh lesser matters. Less likely to be updated or changed than most web pseudopapers, as errors of fact and interpretation are much less likely.
6. Papers, electronic (not electronic versions of journal papers). A serious scholarly contribution to members of the same scholarly community, usually a community of higher learning, but also accessible to an informed lay audience. It usually presents results of an original study or argument. The data, methodology, and results are reviewed by respected scholarly peers in order to make sure the work is original, the consclusions drawn are supported by the results, the arguments are strong and coherently presented, and sufficient data has been collected to put forward a meaningful result, all 'on the fly' by circulating the paper widely for comment. The paper may ultimately be electronically published with numerous appended critiques and commentary.
7. Reviews. A synthesis and synopsis of the current views of a subject written by scholars in the field and edited by respected scholars in the field. Includes opposing views, historical perspectives on the movement in ideas in the subject, obscure facts, unpublished data, relevant and pertinent references to journal papers. Reviews can sometimes show biase, include shaky data, and heavily lead an arguement in a certain predetermined direction.
8. Papers, journal. A serious scholarly contribution to members of the same scholarly community, usually a community of higher learning. It usually presents results of an original study. Prior to presentation, the data, methodology, and results are reviewed by respected scholarly peers in order to make sure the work is original, the conclusions drawn are supported by the results, the arguments are strong and coherently presented, and sufficient data has been collected to put forward a meaningful result.
and 'soft' or 'fuzzy' science
Hypotheses which can be tested by chemical and/or mechanical experiment over and over again, always with the same result, are the chief way of investigation in chemistry and physics and closely allied disciplines. These are generally referred to as 'hard sciences', as they general 'hard facts', data which is gained from replicable experiments, and which endures due to it involving the fundamental laws of chemistry and physics.
Hypotheses rely on incomplete data sets, partial sampling, observations of the behaviour of living organisms, interpretations of the environmental residues of behaviour, and divining the use of a physiological or morphological feature in an organism from its use in another related organism is often regarded as 'soft' or 'fuzzy' science. There is an almost unspoken hierarchy of 'worth' between 'hard science' at one end of the continuum, and the most 'fuzzy' sciences at the other end -
All we can do is use the sparse information science can provide, noting that cladistic analysis is unreliable, sampling of African fossils hugely biased to relatively small geographically limited 'fossiliferous areas', and attempt to develop plausible scenarios which can never be proven, only disproven. Peering back millions of years and trying to suggest a 'path' of evolution of the human diet is necessarily entangled in the whole question of how human animals evolved. We will never know for sure, but we can invent fact-based plausible arguments for the detail, and highly probable arguements for the process. But paleo-anthropology of the human evolution within various feeding ecologies will always be fuzzy science. This may be irritating for those who need the security of the rigor of 'hard science', but these are the unalterable terms for engagement in discussion of this field of knowledge. In spite of having to step into an arena where there is necessarily speculation, extrapolation, thinly supported evidence, concatenated arguments, lack of data and profound ignorance of wild human behaviours, the question of human evolution is both fuzzy and intellectually compelling -
There are relatively more recent fossils of 'ape-like' animals, placed mostly in the genus Australopithecus - "Southern apes". (A newly discovered ape-like animal with a chimp size brain case but apparently flatter face has been placed in a new genus, Kenyanthropus.) There are fossils of these animals spanning from about 6 million years ago to the time when we can be substantially confident we are seeing the emergence of early humans - about 1.8 million years ago, with fossils of Homo erectus.
If we look for information from the bones and artifacts of these animals, we have to ask, first, are these bones the fossilised remains of the ancestor of any extant hominoid? If so, which hominoid? Chimpanzee? Human? Gorilla? Secondly, we have to ask - if we can confidently show that the Australopithicenes were ancestral to Homo erectus (and thus to Homo sapiens), which particular Australopithicine was directly ancestral? Were more than one species directly 'in the line' to Homo? Thirdly, we have to ask if we have even found any fossils of any animal ancestral to any of the three living African Hominoid genera at all.
Considering the first question, we have to admit that as humans, we are biased in trying to explain ape-like fossil. A dispassionate view would say the fossils are of animals that might be described as 'ape-like' - and therefore ancestral to present African apes. On the other hand, they are not just ape-like, but also largely bipedal - and therefore ancestral to humans. We have a tendency to assume that fossils found are part of the explanation of human origins. They may equally be part of the explaination of ape origins. And they may equally be evolutionary dead ends, leading to no existing ape or human. So we have to be cautious about interpreting a given fossil as if it were ancestral to human. It may be ancestral to Chimp, Gorilla - or none of us.
Addressing the second question, if an Australopithecine was in fact ancestral to humans, we have to ask, which one? We can reject any notion that we can demonstrate with any certainty an evolutionary relationship between these fossil specimens or between any fossil specimen and living African Hominoid. Fossil skulls are unreliable in proving relationships[n]. Fossilised body parts below the head are more reliable - but are often fragmentary. 'Cladistic analyses' on these bases must be regarded as suspect. There is no way of knowing which shared physical features in Australopithicenes, Chimpanzee, and Gorilla are derived from a common ancestor, or simply adaptations evolved in parallel. Further, so few whole bones are known that a useful idea of the variability withinin a species, let alone subspecies, cannot be formed.
The final question is about the extent of sampling of the fossil record. We have to accept that the 'sampling' of fossils in Africa is totally inadequate to gain an idea of what kind of ancestral animals were there, where they lived, and for how long [n]. Fossils have been found mainly where huge rifts in the ground have exposed ancient strata, or where limestone caves or holes exist where animals might fall and fossilise. In addition, the soils must contain alkalai componds such as calcium carbonates and phosphates to mineralise and preserve bone. These special conditions self limit mainly to the rift valley system and soils of Kenya, Tanzania. Ethiopia, and a few favorable sites in Southern Africa (in limestone formations, or ancient sand covered inland fossil beaches). Huge areas of Central and Southern Africa yeild no fossils, both because the soils are leached and acidic (in lateritic soils beneath rainforest, typically pH 4.5 to 5.5) and destroy bone, and because what fossils might be there are deposited in strata deep beneath the ground and not exposed by rifting of the earth's crust. These problems are not unique to African hominoids. One estimate (Martin 1993) is that only 3% of extinct primates have been documented, and the problem of biased preservation is at least as bad for primates in general as for apes, with consequences for primate evolutionary inferences.
However, it would be foolish to ignore what inferences we can draw from the strands of evidence from extinct Hominoid fossils; while remaining mindful of the dangers of too confident extrapolation. Other strands come from comparison of all elements of the ecology of the three extant African Hominoids, and trying to relate diet to physiology, masticatory systems, morphological limitations, and habitat food sources. Again, we must be mindful that the morphology of Chimpanzee and Gorilla has been moulded over a long evolutionary period within a tropic and subtropic forest environment. Just where our morphology has been shaped is debatable, and part of the purpose of this pseudopaper
Hominoids an unecessary name for all the members of of the super-family Hominoidea, that is, all extinct and present apes - lesser apes (gibbons, family Hylobatidae), great apes (orangutans, gorillas and chimpanzees, family Pongidae) and the human ape and its extinct bipedal relatives (family Hominidae).
Hominid a term for all extinct and present bipedal apes, that is, the members of the family Hominidae - it takes in all Australopithecines (species in both the genera Australopithecus and Paranthropus) and all species in the genus Homo
The fossil record so far sampled - on line to human,
on line to Chimpanzee, on line to neither?
The following is a brief exercise in both constraining and allowing possibilities, and is not in any way exhaustive. It does illustrate the need to suspend judgement and maintain healthy skepticism when trying to allie the dentition or other aspect of extinct hominid morphology to evolution of human feeding ecology.
Estimating a time for a large bodied hominoid species to formally speciate.
Ruovolo estimates a range of 300,000 years to 2.8 million years between divergences of large bodied hominoids. Therefore, if we use the 300,000 year 'rapid speciation' end of her estimate, then it is possible that 'Australopithecus' rudolfensis (fossil KNM-ER 1470 dated to about 2.4 million years ago, assigned to the genus Australopithecus by some, the genus Homo by others) commenced to form as a species from about 2.7 million years ago. Of course, these are post fact expainations. Speed of speciation may have much more to do with exceptional circumstances, such as increasing aridity, causing sudden radiations, modified by differing local conditions in different latitudes, altitudes, and proximity to dispersal routes versus 'peninsular' effects.
Likewise, using the 2.8 million year 'slow speciation' estimate, then it is possible that Australopithecus rudolfensis commenced to form about 5.2 million years ago.
An 'in-between' scenario would allow an ancestor to A. rudolfensis at about 4 million years ago. For the sake of brevity, it will not be developed. (While neither scenario would exclude Kenyanthropus platyops as a direct ancestor of H. rudolfensis; in the 'slow speciation' scenario rudolfensis would have to arise as a local bud, and the Kenyanthropus stem continue on, both presumably sharing the same niche - possible, but improbable.)
These scenarios have an assumption of a woodland, more or less bipedal, chimpanzee sized animal as our ancestor, with no fossils as yet found. For a 'postulated' ancestor to be unsampled, it could be argued that it most likely would have had to have evolved and lived west of the Rift valley, in soils unsuitable for fossils, or in the humid tropical forest - also with soils unsuitable for fossils.
As humans and chimpanzees share a common ancestor, we have to explain the evolutionary pathway of chimpanzee as much as we have to explain our own. Chimpanzees are geographically confined predominantly to tropical forests. Chimpanzee ancestors were not necessarily confined to tropical forests. At least, not necessarily confined prior to the emergence and successful expansion of Homo or Homo-like animals.
Evolutionary distance between chimpanzee and
ancestral Homo lineage - fast and slow speciation
Fast speciation scenario - fossil based
If humans and chimpanzee share a common ancestor - keeping in mind that the date of existence of the last common ancestor is not (necessarily) the date of speciation - then if A. rudolfensis is formed in a 300,000 year period of 2.7 to 2.4 million years ago, chimp could be derived from the common ancestral line up until 2.5 mya - the closest reasonable approach to the time A. rudolfensis might formally appear as a species. Chimpanzee could have branched off at any point back to an undetermined time, as we don't know which species was the last common ancestor, and we don't know whether it existed for hundreds of thousands of years as a species, or millions of years.
As chimpanzee is a tropical jungle animal, we must associate its ancestry with fossils in Kenya/Tanzania, thus excluding southern Africa's A. africanus as ancestral to chimp (unless A. africanus fossils are discovered near the equator). A. garhi must be excluded as being too close to 2.4 mya to be a chimpanzee ancestor. In other words, based on sampled fossils, the human/chimp LCA could be either A. afarensis or Kenyanthropus platyops at 3.5 to 3 mya, it could be A. anamensis at 4 mya, it could be the other recently discovered hominid, Orrorin tugenensis, at 6 mya [r] . Some of the possibilities include-
1.Chimp could derive directly from A.
anamensis; A. rudolfensis also so derived, but
intermediate fossils unsampled as yet; A. afarensis also
derived from A. anamensis, but ultimately the afarensis
branch is a dead end. (Although parallel evolution in two somewhat
similar lineages - a lineage toward Homo and a separate
lineage toward A. afarensis/A. africanus) is always a less
simple explanation, and generally a second option, it cannot be
ruled out. Orang-utan, for example, has a somewhat similar pelvis
to African hominoids. Although a more ancient lineage, it also
evolved in a tropical jungle, and evolved similar pelvic solutions
to the lifestyle in parallel with African apes.)
2.Chimp could derive directly from A. anamensis; while A. rudolfensis is a bud off either A. afarensis at 3 to 2.9 mya or A. africanus at around the same time interval; A afarensis also derived from A. anamensis, but again, ultimately A. afarensis and A. africanus branch is a dead end.
3.Chimp could derive directly from Orrorin tugenensis after 6 mya, intermediates unsampled; A. rudolfensis also so derived, Kenyanthropus platyops an intermediate; A. afarensis also derived from Orrorin tugenensis, but ultimately this branch is a dead end.
4.Chimp could derive as a bud off A. afarensis 3.6 to 3 mya, A. africanus also; A. rudolfensis an afarensine bud as well.
5.Chimp could derive as a bud off A. afarensis 3.6 to 3 mya, A. rudolfensis already having split earlier from A. anamensis, but intermediates unsampled.
6.Chimp could derive as a bud off A. afarensis 3.6 to 3 mya; A. rudolfensis already having split earlier from Orrorin tugenensis, but intermediates either unsampled or Kenyanthropus platyops the intermediate.
7.Chimp could derive as a bud off Kenyanthropus platyops at 3.5 mya; A. africanus and A. rudolfensis derive from A. afarensis 3.6 to 3 mya
Fast speciation scenario - entirely speculative, no fossils exist
1. Chimp could be derived from an unsampled equatorial jungle semi arboreal, semi terrestrial, quadriped 'sometime' before 4 million years ago; the lineage to Homo also so derived via A. afarensis .
2. Chimp could be derived from an unsampled equatorial jungle semi arboreal, semi terrestrial, quadriped 'sometime' before 4 million years ago; the lineage to Homo also so derived via still unsampled intermediaries; A. afarensis and sequencial species also so derived, but going extinct without issue.
3. A seasonal tropical forest living ape like Oreopithecus at 11 million years ago gives rise to an unsampled animal ancestral to both an open woodland ape such as Orrorin tugenensis, and an unsampled, more bipedal early form of Pan around 8 mya. Pan gives rise to various marginally omnivorous frugifolivorous closed woodland Australopithecus species, all of which go extinct without issue. Orrorin tugenensis, or an unsampled derivative, is ancestral to Kenyanthropus platyops or similar animal, which in turn is ancestral to Homo rudolfensis.
Slow speciation - fossil scenario
As an ancestor 'from which to diverge' is required at 5.2 million years ago, we can look only at two candidates on current fossil evidence - millenium hominid, Orrorin tugenensis, and Sahelanthropus tchadensis, at about 6 million years ago. This constrains the arguments to-
1. chimps diverge somewhere before 5.2 million years ago (permissive on some DNA data molecular clock dating arguments to 10 million years ago [r] ), with no intermediate fossils on the line to chimp sampled to the present day. No fossil grades have yet been found covering the intervening period - from Orrorin tugenensis/Sahelanthropus tchadensis or derivative at 5.2 mya, to Homo rudolfensis at 2.4 million years ago. Alternatively, Kenyanthropus platyops, at 3.5 mya, is such a grade.
2. given H. rudolfensis is (so far) found outside tropical forest, semi bipedal or bipedal Orrorin tugenensis/Sahelanthropus tchadensis gives rise to A. afarensis sometime before 6.4 million years ago. (If A. anamensis is included in the direct line of descent and is considered an intermediate rather than a speciation event itself, the date remains the same. But another 2.8 million years is added if A. anamensis is considered a seperate species - giving 9.2 m.y.a.as the divergence date, and allowing a further, unsampled, species other than Orrorin tugenensis/Sahelanthropus tchadensis to be ancestral). H. rudolfensis then derives as a bud off the A. afarensis lineage.(Allowing a similar A. afarensis intermediary for chimp is possible, but takes us back to scenario 4)
Slow speciation scenario - entirely speculative, no fossils
1. chimps diverge about 8 million years ago (permissive on some DNA data molecular clock dating arguments to10 million years ago [r] ), from an unsampled animal ancestral to Orrorin tugenensis/Sahelanthropus tchadensis. No fossils of the tropical forest living ancestral chimp are found; all Australopithicine woodland chimps go extinct without issue. On the human side, the same 'unsampled animal ancestral to Orrorin tugenensis/Sahelanthropus tchadensis' gives rise to H. rudolfensis. Again, either no fossils have yet been found covering the intervening period - from 5.2 to 2.4 million years ago - or Kenyanthropus platyops is one such 'rudolfencine' intermediate fossil grade .
Fit with Molecular clock and mtDNA data
DNA divergence data sets predicated on thinly supported last common ancestor date assumptions allows a chimp divergence in the range 3.1 to 5.8 mya.
Protein differences between species puts divergence of apes and humans at 'about' 5 million years ago, but are also rooted in thinly supported intra-ordinal primate divergences.
Most mitochondrial DNA (mtDNA) dates have an anchor point for calibration of their molecular clock of 5 million years ago, i.e., the last common ancestor of chimp and human (usually a chimp-human dichotomous split is assumed) is 'assumed' to be 5 million years ago. This is in spite of the absence of fossils of a common ancestor to support the assumption. The assumed time of the split, a 'doctrine' unchallenged for 30 years, calibrates the mtDNA clock, but the 'assumed' timing is not supported by recent mtDNA work anchored in inter-ordinal mammalian splits with actual supporting fossil evidence [r]. So mtDNA data must be regarded as relatively uninformative at this point.
The fossil record is sparse and localised, we cannot know the trajectory of anthropoid evolution, whether from riparian woodland to tropical forest or vice versa; whether from biped to quadruped or vice versa. Or none of the preceding. When we look at an Australopithicene (in its most inclusive sense) we don't know if it is an ape ancestor, a human ancestor, or if it is an evolutionarly dead end, with no proceeds.
We can speculate on a plausible evolutionary trajectory that is able to explain the human ecological feeding niche by constraining plausible feeding ecologies, morphology, and behaviours with as many lines of (necessarily weak) evidence as can be found. One useful constraining tool is to posit the existance of a large bodied hominid in each distinct African feeding ecology and then attempt try ancestral humanimals in such ecologies to see if they 'fit', knowing what we do about our digestive anatomy and our physiological needs. Whether the bones of such animals have been found is moot.
Occam's razor and the sample of hominoid fossils - scientists generally prefer the simplest explanation to account for an observed 'data set'. Generally, a 'working hypothesis' accounts for the observations until it is shown to be false or null. Attempting to form 'hypotheses of human evolution' from a small set of fossils which are confined to a tiny portion of the African and adjacent landmasses is an inevitable result of our need to know our origins. It is only a slight exaggeration to say that as soon as a new hominoid fossil is found, it is claimed as a human ancestor... but all hypotheses based early fossils must be viewed as intrinsically weak due to biased sampling giving an incomplete and possibly unrepresentative data set. More complex, non-parsimonious hypotheses based on multiple strands of evidence and analogue are not much weaker, and therefore ought be given more weight.
Genetic relationship of Gorilla, Pan, and Homo - the earliest attempt to establish the relationship of Gorilla, Pan and Homo was by Sibley and Ahlquist,1984 reported in the 'Journal of Molecular Evolution'. The technique used was 'DNA hybridisation'. After correcting for reported data errors, they found that their data was best explained by a more or less trichotomous split of Gorilla, Pan and Homo at about the same time from the common ancestral species. If this techniques assumptions are correct, it suggests that Gorilla, Pan and Homo are genetically about equally distant from each other (with about 2.3% difference in the DNA of Gorilla, Pan and Homo - not the widely reported 1.8% difference between Pan and Homo exclusive of Gorilla), and all split from a common ancestor at around the same time.
Other reviews of DNA data sets - Ruovolo 1997 [r] in the journal 'Molecular Biology and Evolution' being the most complete to date - find, on balance, that Pan and Homo are most closely related, and Gorilla is less similar.
Fossil specimens cannot be arranged into related groups with any certainty - this follows the 'Biological Species Concept' of Mayr, which accords greatest weight to interpopulational gene-flow in the wild.
sampling - Sampling starts with
approaching a 'lot', the entire 'population' or 'set' of things,
among which is all variation, great or small. The actual number of
samples that need to be drawn out of the lot to represent 95% of
the variation contained within the lot (the level of effective
'certainty') can be derived by various statistical formulaic
methods. One author claims a minimum sample of 40 to describe a
dominant character suite in a species. Livestock scientists
commonly regard 80 animals as the minimum to avoid chance extremes
in phenotype misrepresenting the 'typical' phenotype of a given
population under study. If human evolution from last common
ancestor to Homo erectus is regarded as the 'lot', then
the entire range of the various species must be known, fossils of
each of the species must fortuitously exist, and sufficient
fossils exist to represent the range of variation within
each species. None of these conditions exist - and can never
We can never attain 95% certainty that we have sampled and 'know' the course of human evolution.
Martin RD, Soligo C,
Will O, Marshall CR, Tavaré S. 'Early primates and the
effects of a fragmentary fossil record on dating evolutionary
divergences' note in the Symposium Abstracts of an International
Meeting on the Evolution of Vertebrates in Lund, Sweden in 1999
[available at: http://phylo.gen.lu.se/Symposium_Abstracts.html
"...The earliest known unequivocal fossil primates come from basal Eocene deposits (about 55 Mya) and the standard view is that primates originated about 65 Mya. A similar conclusion has been reached for most orders of placental mammals, and it is widely accepted that the origin and radiation of many mammalian groups occurred following extinction of dinosaurs at the end of the Cretaceous... All of this reflects the common procedure of dating the origin of a group by the first known fossil representative, perhaps adding a few million years as a safety margin. Such direct dating from the fossil record faces 2 problems: (1) If the fossil record is very fragmentary, the first known fossil representative is likely to postdate the actual origin by a substantial margin. (2) Potential sources of bias in the fossil record may introduce a further margin of error. This has direct implications for the widespread practice of calibrating molecular trees with a single date for the first known fossil of a group. (Here, it is important to note a distinction between the time at which a given group diverged and the time at which its diversification began.) Using a simple approach, Martin (1993) calculated that only 3% (at maximum) of extinct primate species have so far been documented, and poor sampling is also evident from the fact that the discovery rate for new fossil species is still accelerating. Rough correction for resulting underestimation of the actual time of origin led to the proposal that primates originated about 80 Mya....calculation based directly on a very fragmentary record is spurious... In fact, there are extensive gaps in the mammalian fossil record generally. Very few fossils are known for the Jurassic and most of the Cretaceous, two thirds of mammalian evolution....Several recent results from analyses of molecular data using a range of calibration dates outside the primates have confirmed an early date for the origin of primates. Inference of divergence times for orders of birds and mammals from nuclear gene divergence calibrated with the well-documented split between synapsid and diapsid reptiles set the origin of primates at about 90 Mya (Hedges et al. 1996; Kumar et al. 1998). Demonstration of an African clade of placentals (Springer et al. 1997) provided further support for early divergence between mammal orders. Calibration of mtDNA-sequence trees with dates for the earliest known (Palaeocene) cetaceans also set the origin of primates at about 90 Mya (Arnason et al. 1996, 1998).
Finally, the problem of bias in the fossil record must be addressed. Modern primates are largely confined to tropical and subtropical forests of the southern continents. Yet the earliest known primates from the Eocene are largely confined to the northern continents and show little overlap in distribution. The most plausible explanation for this is that probabilities of fossil preservation and discovery are far higher in the northern hemisphere and that the record simply reveals a transitional northward expansion of an essentially tropical/subtropical group of primates groups when temperatures were significantly higher during the Eocene. The early fossil history of primates in the southern hemisphere remains virtually uncharted. " [My emphases.]
Central African fossil Australopithicine - 3.0- to 3.5-million-year-old australopithecine jaw was discovered in Chad, in Central African about 1996. Provisionally assigned to Australopithecus afarensis a species known from East African sites at Hadar, Ethiopia and Laetoli, Tanzania. This is the first australopithecine discovered west of the Rift Valley. Paleoanthropologists had always assumed the Rift valley acted as a geographical barrier separating hominid populations. A curious proposition that a semiarboreal ape, of all animals, would be unable to negotiate the valley!
Úlfur Árnason. 1999. 'Temporal aspects of primate divergences'. Symposium Abstracts, International Meeting on the Evolution of Vertebrates in Lund, Sweden. In this symposium presentation Árnason suggests that the proposal by Goodman, Sarich, and Wilson in 1967 the divergence between Gorilla, Pan (chimpanzee) and Homo had taken place 5 million years ago is incorrect. This was based on the molecular distances between the three genera being about 1/6 of the distance between the members of Cercopithecoidea (e.g.. baboons, macaques) and Hominoidea (Gorilla, Pan and Homo). The assumption was that the time of divergence of Cercopiths and Hominoids was 30 million years ago. This date was based in turn on the fossil evidence available at the time. The interpretation, and therefore timing, of this fossil evidence has since been questioned. In addition, rooting the calculation in groups with a better known fossil evident divergence, results in a recalculated Cercopithecoidea - Hominoidea suggested divergence at 55 million years ago, and consequently, a Pan and Homo divergence at least 10 million years ago. While this is unlikely, it shows the uncertainties in the anchor point used to calibrate divergence times.
Shorter versus longer scenarios for divergence of apes from a common ancestor - Longer scenarios are more 'attractive' in allowing time for major morphological differences, for example, between the australopithicene and both the ape and the human pelvis - both are equally specialised - to develop; but intensive selective pressure might well allow rapid evolution (we could say 'backward', but this is false bias to a predetermined direction for evolution) of ape pelves enclosing a folivorous gut, a guts full of low energy value plant food that is in turn supported by four limbs rather than two.
sagittal crest (sagittal keel): A crest of bone running along the midline of the skull (like the keel of a yacht) for the attachment of enlarged temporalis muscles that meet along the midline of the skull. The temporalis muscles give the power to the bite. Large crushing forces require large muscles and large attachments for the muscles.
The tropical rainforest biome
Both tropical evergreen and semi-deciduous forest ape plant food data are available from primatologist research teams such as those of Idani, Wrangham, and others as text delineated files for Mahale, Lope, Wamba, Gobe, Kahuzi, Ndoke, Ugalla and Asserik. Unfortunately I omitted to note the URL.
Western and Eastern Gorilla food plants - the various studies show a wide range in categories contributing to the total diet. For the Western gorilla the range is 17%-48% frugivory, for the Eastern 9%-47%. Likewise for other categories. 6%-34% tree leaf folivory for the Western Gorilla, and a range of 17%-51% for the Eastern gorilla. These variations may reflect the time of year the study was made, the averaging of lengthy studies, and the habitat 'patch' the observed animals were in at the time. See Conklin-Brittain et al for details.
Miocene era - from 23 million years ago to 5.5 million years ago
Chimpanzee hunting with tools - On rare occasions chimpanzees have been observed using 'tools' to help catch prey. Lacking a firm 'stance' and having a torso and limbs adapted for both climbing and knuckle walking, they cannot apply well directed force.
Miombo: dry woodland in southern Africa dominated by species of the genera Brachystegia, Isoberlinia and Julbernardia
Availability of the
two essential fatty acids from food as a bottleneck to brain
expansion - Long chain polyunsaturated fatty acids -
'LCPUFAs' - and particularly docosahexaenoic acid (DHA), are
essential for human brain tissue development. The LCPUFAs
eicosapentanoic acid (EPA) and its conversion product, DHA are
abundant in some lake (East African lake fish 549mg/100gms [r])
and sea fish. The only abundant dietary source of these preformed
LCPUFAs on land is brains (861mg/100 gms), arguably an irregular
feature of the diet. Some have argued that only when we exploited
coastal foods did we regularly receive enough of these pre-formed
fatty acids involved in brain development to 'allow' selection for
a larger brain.
But pre-formed LCPUFAs are not needed. High levels of LCPUFAs for
human brain development are actively synthesized in the liver [r]
from the precursor fatty acids linoleic acid (LA, omega-6,
converting mostly to arachidonic acid 20:4) and alpha linolenic
acid (ALA, omega-3, some converting to EPA, and then DHA, 22:6)
under two conditions. First, there is no excessive supply of
linoleic acid (18:2, omega-6) relative to alpha linolenic acid
(18:3, omega-3) acid. Second, 3 chain fatty acids are present in
the diet. Both conditions are met in a 'wild' omnivorous diet of
plant foods and a little animal food. A shortage and imbalance are
difficult to arrange in a mosaic feeding ecology.
"The predominate fatty acids in these wild foods [foliage and
fruit in neotropical America] are palmitic (30%), linoleic
(23%) alpha linolenic (16%) and oleic (15%). Fatty acids
with less than 16 or more than 18 carbon chains are uncommon.
(range 0 to 7%). Saturated and unsaturated fatty acids are almost
equally balanced. These wild foods also contain a high percentage
of both omega-3 and omega 6 fatty acids" (Chamberlain, Nelson and
In addition, eye and brain tissue can also synthesize one of the LCPUFAs (DHA) given the availability of the precursor [r]. There is evidence that as fatty acids transfer from maternal liver to pancreas to the foetal liver and finally foetal brain, there is progressive transformation, in the form of increased length and reduction in saturation [r]. This is in addition to the incorporation of any LCPUFAs assimilated by the mother directly from the diet [r]. Babies are able to not only absorb pre-formed LCPUFAs, but also synthesize DHA themselves from precursor fatty acids in the breast milk [r].
The bow and arrow food harvesting tool - Cave paintings dated to 12,000 years ago confirm the use of this tool at this time, but whether developed or not, it is not universally used. Aboriginal people don't use this technology, even although there are venomous snakes in Australia from which to extract poison. Some suggest the idea that arrow points may have been used much earlier [r]. Small projectile points dated to at least 90,000 years ago have a tang, which suggests they were hafted to an arrow shaft.
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