"Life on Earth began more than 3 billion years ago, adapting and evolving over time, from the most basic kinds of microbes, into complex Humans and other animals. But how did the first organisms adapt and evolve on Earth, which is the the only known home to life in our universe develop from the primordial soup?
Once upon a time, a group of atoms gathered in an idle life-like structure in a warm pool of water and absorbed energy from the sunlight and began dissipating energy from the idle life-like structure in increasing amounts …. gathering and dissipating …. gathering and dissipating …. and wham-bam …. that idle life-like structure began living. (A New Physics Theory of Life, by NatalieWolchove Jan 22, 2014 Quanta Magazine)
“The first known single-celled living organisms appeared on Earth about 3.5 billion years ago, roughly a billion years after Earth formed.” (How Did Multicellular Life Evolve?, by Charles Q. Choi, Astrobiology NASA)
“All multicellular organisms, from fungi to humans, started out life as single cell organisms. These cells were able to survive on their own for billions of years before aggregating together to form multicellular groups.” (Enrico Sandro Colizzi Renske MA Vroomans Roeland MH Merks (2020) Evolution of multicellularity by collective integration of spatial information)
The Origin of Human Life is an extremely emotional topic. The debaters and audiences fall into 2 major categories i.e. people who believe and worship God and people who do not. Every Human in the United States is free to do either. This dissertation is not a recruitment process for one or the other. To believe and practice a religion, spirituality and/or meditation or not to believe and practice are irrelevant herein.
There are 2 additional Human groups, who weigh-in on this discussion i.e. Scientifically Literate Humans and Scientifically Illiterate Humans. And there are additional mixtures of the 4 groups.
Needless to say, during the COVID-19 (Coronavirus) Pandemic, most Humans have embraced the Scientific and Medical baseline explanations for the horrific Pandemic circumstances i.e. sudden illness, sudden deaths, unemployment, business closures, economic loses and all other catastrophes.
And as is the most often case, many Humans turn-to Religion, Spirituality and/or Meditation beliefs and practices, seeking and praying for God's blessings, grace, mercy and healing.
So when it comes to the equally serious topic, ' THE ORIGIN AND EMERGENCE OF 1ST LIFE AND THEN HUMAN LIFE', Humans are best advised to embrace the fact based evidence from Scientific and Medical research, conclusions and explanations, which this MINIX CLINICIAN, ANALYST AND REPORTER's dissertation intends to dissertate.
[The Origin and Emergence of Life Under Impact Bombardment, Charles S Cockell, Phil. Trans. R. Soc. B3611845–1856 http://doi.org/10.1098/rstb.2006.1908] follows:
“Craters formed by asteroids and comets offer a number of possibilities as sites for prebiotic chemistry, and they literally invite the application of Darwin's ‘warm little pond’.
{{{ Charles Darwin said,“But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts, light, heat, electricity etcetera present, that a protein compound was chemically formed, ready to undergo still more complex changes [..] " ~Charles Darwin, in a letter to Joseph Hooker (1871)
“All life on earth is related. Trace back the separate lines of descent of all organisms that ever lived, and they will converge to a single point of origin - the beginning of life. Charles Darwin was reluctant to publish his views on life's origin. His only speculations on the subject are known from a private letter to his friend and colleague Joseph Hooker, in which he speaks of a 'warm little pond' in which the first molecules of life could have formed.
“This new hypothesis brings the origin of life debate back from the depths of the oceans to the surface of the earth. Other scientists believe hydrothermal vents in the deep sea are the most conducive environments for nascent life.
“The researchers, led by Armen Mulkidjanian, presume that the chemistry of modern cells mirror the original environment in which life first evolved. Since oceans and cells are chemically dissimilar, they think it is unlikely life evolved there. The chemical nature of volcanic pools, or 'warm little ponds', resembles the cell's composition of its cytoplasm much more closely.
“The researchers invented the term 'chemistry conservation principle' for their idea that organisms retain their chemical traits throughout time. Cells evolved ion pumps and ion-tight membranes to maintain the ion balance that was initially forced upon them; hence the assumption that cells themselves are reflections of their ancestral environment.
“This is not a new approach. The Canadian biochemist Archibald Macallum applied it as early as 1926, when he noted that ion levels were similar between blood and sea water and concluded that animals must have evolved in the sea. "Maccallum was also the first to measure the concentrations of ions within cells", says Mulkidjanian. "He discovered that all modern cells contain more potassium than sodium."
“This century old observation is one of the cornerstones of Mulkidjanian's argument: potassium outnumbers sodium in living cells, yet in oceans and lakes, sodium dominates. Other ions, like zinc, magnesium and phosphate are also present in much higher concentrations in modern cells than they are in oceans of past and present.
“The same small set of ions is built into the core machinery of the cell, inherited from the last common ancestor of life. The backbone of DNA is made of phosphate, many ancient proteins require zinc, and the cell needs potassium ions to connect amino acids together in the manufacture proteins, one of the most important chemical reactions in life.
“From these observations, Mulkidjanian and his colleagues conclude that it is unlikely life evolved in the sea. They think terrestrial springs, like those in Yellowstone Park, are much better candidate environments for the earliest evolution of life.
“They argue that geothermally active pools are the only places on earth where potassium, zinc, magnesium and phosphate are found in high enough quantities to explain the ionic content of cells.
[Mulkidjanian, A., Bychkov, A., Dibrova, D., Galperin, M., & Koonin, E. (2012). PNAS Plus: Origin of first cells at terrestrial, anoxic geothermal fields Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1117774109][Did life evolve in a `warm little pond'? By Lucas Brouwers on February 16, 2012, Scientific American. }}}
Life on our planet, Episode 1 and 2 Netflex Series offfered the following:
- Competition drives adaptation and evolution
- Earth never remains stable alone.
- Our changing Earth has caused many changes in life
- stable Earth facilitated Dinosaurs era
- After asteroids struck 66 million years ago 5th mass extinction of Life and dinosaurs extinction 66 million years ago and destroyed 75% of life
- Water and sunlight energy supported life beginning with 1st 1 cell life
- Competition drives adaptation and evolution
- All life can trace back 4 billion years.
- 99% of original species that trace back 4 billion years when life began, are now extinct.
- 530 million Years ago jelly fish were the first animals to escape the sea floor and swim escaping Predators
- Green house gas dropped its CO2 concentration and Ice age lasted 200;000 year and 85% of all life
- Deep ocean remains unchangsd ; below 600 meters temperature doesn’t change where animals hid from ice age
- Only 10% of fish today live in deep ocean water
- 440 million years ago CO2 bounced back and ice melt began. Fish swam back up out of deep as ice melted
- Now 30;000 species of fish
- Backbone give fish speed and power. Jaws give them bit
- Unchanged since first melt Sharks speed, power and Jaws facilitate their attack in teamwork to corral prey up into ocean shallows
- Second mass extinction 80% of marine life wiped out
- Plants began greening planet Earth and greenery and oxygen paved the way for animals to follow and survive
" Scientists said they would not be surprised to discover that animals first crawled out and adapted to land as early as 470 million years ago, about the same time that plants were taking root there. Indeed, some fossil finds made recently in Pennsylvania, but not yet reported in science journals, may prove to be of terrestrial animals that lived more than 430 million years ago. Maybe 414 years ago. [ A version of this article appears in print on Nov. 3, 1990, Section 1, Page 9 of the National edition with the headline: Did Life Leave the Sea Earlier Than Thought? from "Did Life Leave the Sea Earlier Than Thought?By John Noble Wilford Nov. 3, 1990 NY Times]
“Some of these attributes, such as prolonged circulation of heated water, are found in deep-ocean hydrothermal vent systems, previously proposed as sites for prebiotic chemistry.
“However, impact craters host important chemistry characteristics in a single location, which include the formation of diverse metal sulfides, clays and zeolites as secondary hydrothermal minerals (which can act as templates or catalysts for prebiotic syntheses):
- · fracturing of rock during impact (creating a large surface area for reactions)
- · the delivery of iron in the case of the impact of iron-containing meteorites (which might itself act as a substrate for prebiotic reactions)
- · diverse impact energies resulting in different rates of hydrothermal cooling and thus organic syntheses
- · and the indiscriminate nature of impacts into every available lithology:
- · generating large numbers of ‘experiments’ in the origin of life.
“Following the evolution of life craters provide cryptoendolithic and chasmoendolithic habitats, particularly in non-sedimentary lithologies, where limited pore space would otherwise restrict colonization.
“In impact melt sheets, shattered, mixed rocks ultimately provided diverse geochemical gradients, which in present-day craters support the growth of microbial communities.
“During the Hadean, (in Geology Hadean denotes the period when the earth was forming; the surface of the Earth was subjected to relatively heavy bombardment by asteroid and comet impacts, compared to the surface of the present-day Earth (Nisbet & Sleep 2001; Kring & Cohen 2002; Ryder 2003).
“Assuming that the origin of life occurred on Earth, it happened in an environment that was periodically disturbed and subjected to relatively heavy bombardment by asteroid and comet impacts
“The impact flux by asteroid and comet impacts on the early Earth was not only higher, but the frequency of large asteroid and comet impacts with extremely high-energy were much higher then. Although the impact events cause environmental disturbance, the craters associated with them might have characteristics that make them the suitable geochemical environments for prebiotic chemistry, particularly with respect to their hydrothermal systems.
Impacts affecting the origin of life can have occurred in air, land and sea. However, hydrothermal vents on the ocean floor have received much attention as one of the sites for the origin of life (e.g. Wächtershäuser 1988; Holm 1992; Russell & Hall 1997). These environments, mutatis mutandis, might provide a basis from which to investigate the potential of impact-induced hydrothermal systems as sites for prebiotic chemistry.
Following the emergence of life, impacts would have had a profound influence on its distribution, characteristics, adaptation and evolution. Between the time of the formation of the Earth and ca 3.8 Gyr ago {Gy, Ga ("giga-annum"), Byr and variants. The abbreviations Gya or bya are for "billion years ago" (3.8 Byr ago) [Yarus, Michael (2010). Life from an RNA World: The Ancestor Within. Boston, Massachusetts: Harvard University Press. p. 38. ISBN 0-674-05075-4.]}
Life in the oceans may have been periodically destroyed, if it existed at this time (Sleep et al. 1989). Impactors larger than 500 km in diameter have been suggested to have sufficient energy to boil the whole ocean (Sleep et al. 1989) or to have at least boiled the top few hundred meters (Ryder 2003). These events would have favored life in the deep regions of the Earth's crust below the oceans.
The periodic global heating caused by impacts has been suggested as an explanation for the hyperthermophilic (love or high temperatures) root of the phylogenetic tree of life (Maher & Stevenson 1988; Sleep et al. 1989).
It is apparent, based on the study of extant microbial communities within impact-altered target lithologies, that craters would not only have disturbed the early biosphere, but would have provided a suitable, and in some cases improved, geomicrobiological environment for early life. Certain lithological changes induced by impact would have improved the conditions for lithophyte (grow on bare rock or stone organisms (Cockell et al. 2002).
A diversity of geochemical environments, air, land and sea (air means as asteroids and comets trajected toward Earth they hypothetically gathered dust exploded back into outer space with the ingredients for life from previous impacts against Earth and returned ingredients for life to Earth a second time). The 3, air, land and sea have been proposed as suitable sites for pre-organic synthesis and the origin of life. Hydrothermal systems have received attention as sites for organic synthesis and the origin of life (e.g. Wächtershäuser 1988; Ferris 1992; Holm 1992; Russell et al. 1998; Simoneit 2004).
A sufficient concentration of reactants is difficult to imagine in the open oceans, where dilution does not favor the required local conditions for prebiotic reactions. However, mineral surfaces, such as clays, can potentially provide templates, surfaces for sorption, and even catalysis of chemical reactions (Goldschmidt 1952; Rao et al. 1980; Cairns-Smith 1982; Ponnamperuma et al. 1982; Ferris et al. 1988; Cairns-Smith et al. 1992; Lahav 1994).
In theories on the origin of life, asteroid and comet impacts generally have been regarded as unfavorable to these early prebiotic syntheses. The suggested migration of acetogenic precursors of life into the deep subsurface (Russell & Arndt 2005) meets the need to escape the hostile surface conditions associated with impact.
However, craters associated with impacts provide many required geochemical conditions that favor prebiotic reactions. Many of these conditions are contemporaneous during post-impact environmental changes. Thus, craters provide a suite of conditions, which, taken together, make them plausible realistic environments for sustained experiments in the origin of life.
A schematic of the processes occurring in a crater is shown in figure 1.
Figure 1 see Reference: Darwin's warm little pond—the impact crater as a prebiotic reactor. Some of the diversity of characteristics of impact structures that make them favourable sites for prebiotic reactions are shown.
“Asteroid and comet impact craters offer some advantages to lithophytic organisms (Cockell et al. 2005; figure 2), and on early Earth, these environments would have provided suitable places for the proliferation and radiation of life (Cockell 2004) once it did emerge.
“The fracturing of rock during impact will greatly increase the surface area available for microbial colonization, although nutrient limitation could limit the degree to which such void space can actually be used (Cockell et al. 2005), particularly in the subsurface.
Figure 2 See Reference: Impact craters as a habitat. Some of the characteristics of impact craters that make them favorable sites for micro-organisms are shown.
“Favorable conditions for the origin of life require that a diversity of factors come together in a single location. Asteroid and comet impact craters offer favorable conditions for prebiotic reactions that are an amalgam of conditions, which have previously been postulated as required for the origin of life.
“The origin of life in the post-impact environment offers an expansion of the possible number of favorable environments in which such experiments may have occurred. The environments described in this research apply to both hydrothermal systems on early cratons and those established on the seabed following impact into the Hadean oceans.
“The chemical and the physical conditions, which arise in and around impact craters, suggest that Charles Darwin's ‘warm little pond’ (Darwin 1871) is applicable to the understanding possible environments for the origin of life and not just symbolic.
“After the origin of life, up to the present day, impact craters offer favorable environments for colonization by lithophytic organisms. Craters offer a localized source of liquid water, mobilization of minerals and carbon and, in melt sheets and suevite, a potentially large diversity of geochemical gradients, particularly where the lithologic target sequence is diverse, subaerial and submerged craters would have offered favorable environments for diverse microbial communities on early Earth.[The Origin and Emergence of Life Under Impact Bombardment, Charles S Cockell, Phil. Trans. R. Soc. B3611845–1856 http://doi.org/10.1098/rstb.2006.1908, Philosophical Transactions OF The Royal Society was founded in 1660 to promote the new experimental philosophy of that time, embodying the principles of Sir Francis Bacon. Henry Oldenburg was appointed as the first secretary to the Society and he was also the first editor of the Society's journal Philosophical Transactions. The first issue of Philosophical Transactions appeared in March 1665 and featured Oldenburg's correspondence with leading European scientists. ]
‘Oldest Swinger in Town – Torrejonia wilsoni’, New Mexico’s Oldest Primate Torrejonia wilsoni
“A partial fossilized skeleton of a very ancient ancestor of humans discovered in north-western New Mexico has revealed that the first primates lived in trees and that they were not obligate ground-dwellers. More complete fossil material shows that the Palaeocene plesiadapiform known as Torrejonia wilsoni was adapted for a life in the trees. The fossil discovery is important as most of the Palaeocene mammals associated with the first primates (Euprimates) are only known from a handful of bones and isolated teeth.
See reference: The Torrejonia wilsoni Fossil Material Indicates an Arboreal Existence. A skeleton composite of Torrejonia wilsoni (NMMNH P-54500). Picture Credit: Royal Society Open Science
The picture shows illustrations of the fossil material of T. wilsoni (specimen number NMMNH P-54500), with the bones and teeth mapped onto a line drawing of the animal.
Getting into the Swing of Things Once the Dinosaurs Had Died Out
It may sound surprising, but one of the first groups of mammals to rapidly diversify and to become more specious after the extinction of the dinosaurs were the Euarchonta (tree shrews, colugos and primates). These creatures have their origins in the Late Cretaceous and with the extinction of the non-avian dinosaurs, within a few million years, a number of new Euarchonta families had evolved. The sediments that form the Early Palaeocene Nacimiento Formation (San Juan Basin, New Mexico), are one of the most important lithological units for fossils of these small mammals.
A fragmentary skeleton of the plesiadapiform Torrejonia wilsoni found in Torrejonian-aged deposits (NALMA – North American Land Mammal Ages), dating to around 62 million-years-ago, indicates that this animal had an arboreal existence. Previously, many researchers had proposed that the plesiadapiforms, an extinct group of primitive placental mammals, close to the ancestry of primates, had been terrestrial creatures. However, unlike most of the fossils associated with this group of mammals, this specimen of T. wilsoni provided scientists with key insights into the animal’s limbs and joints and a subsequent analysis revealed that it would have been at home in the trees.
Please see reference: Illustrations of Typical Plesiadapiforms
Illustrations of typical plesiadapiforms Plesiadapis cookei (centre) and Carpolestes simpsoni (top right). Picture Credit: DMP (Princeton Field Guild to Prehistoric Mammals)
Dr Thomas Williamson (New Mexico Museum of Natural History and Science), one of the authors of the academic paper published today in the on-line journals of the Royal Society Open Science found the fossil material with his twin sons Ryan and Taylor. Teeth associated with the skeleton allowed the researchers to identify the fossil material as T. wilsoni, no easy task as the skeleton was found jumbled up and mixed in with two other mammals, a partial skeleton of Acmeodon secans and an almost complete skeleton of Mixodectes pungens.
Lead author of the study, Stephen Chester (University of New York) stated: “This is the oldest partial skeleton of a plesiadapiform and it shows that they undoubtedly lived in trees. We now have anatomical evidence from the shoulder, elbow, hip, knee and ankle joints that allows us to assess where these animals lived in a way that was impossible when we only had their teeth and jaws”.
In addition, the research team contend that all of the geologically oldest primates known from skeletal remains, encompassing several species, were tree-dwellers. It seems that the plesiadapiforms, the last of which died out in the Late Eocene, had forward facing eyes and relied more on smell than living primates do. Analysis of the skeleton of Torrejonia wilsoni places plesiadapiforms as a transitional group between other mammals and the true primates.
[By Mike| May 31st, 2017|Dinosaur and Prehistoric Animal News Stories, Main Page, Palaeontological articles, EVERYTHING DINOSAUR]
“The ascent of the Mammals began from catastrophes and emeged into opportunities for the Cretaceous Period, from 145 to 66 million years ago. It is the 3rd and final period of the Mesozoic Era, mammals were doing well on the whole.
“Mammals had adapted and evolved and come a developed significantly, since their Triassic debut, with many insect- eating therians, plant-munching multituberculates and gondwa-natherians woven into the food webs topped by big dinosaurs such as Tyrannosaurus.
“But the Mammals fortunes and the success of many other organisms changed in an instant when an Asteroid shot down from the sky, unleashing a cocktail of wildfires, tsunamis, earthquakes and volcanic eruptions that reshaped the earth in a matter of days and weeks.
“These catastrophes and longer-term climatic and environmental changes triggered by the Asteroid annihilated the dinosaurs. Suddenly, the majestic Dinosaurs that had prevailed for more than 150 million years became extinct.
“Other Mammals also felt the extinction devastation. Evidence for their decline was documented by William Clemens of the University of California, Berkeley, and now led by Wilson, which for five decades has meticulously collected fossils from across the extinction interval. The fact based evidence show that many larger mammals and those with more specialist diets became extinct with the dinosaurs.
Among the other mammals that made it through were some of the earliest placentals i.e. those species like us that give birth to relatively well-developed young.
“Research calculated, when distant ancestors diverged from one another based on DNA differences in living species, the common ancestor of placentals evolved alongside the dinosaurs in the Cretaceous.
“After the end of the Cretaceous and Extinction Period these advanced mammals proliferated and split into the major modern subgroups, including rodents and primates.
“With the extinction of Dinosaurs and other mammals, the placentals had opportunities for conquest and conquer and they quickly evolved heavily populating areas for opportunities on Earth.
“Death of the dinosaurs was the spark that ignited the Placental Animal Revolution. Mammals expedited the Dinosaur catastrophe with newly found diets and behaviors that enhanced the survivors in this Postapocalyptic World.
“Brusatte Colleague’s Thomas Williamson of the New Mexico Museum of Natural History & Science has been dissecting these rocks for more than 25 years and has collected many thousands of fossils in precise detail.
“Among the placentals that Williamson has unearthed in New Mexico is a skeleton of a puppy-sized creature, called Torrejonia, that has gangly limbs, long fingers and toes. It lived about 63 million years ago, but when looking at its graceful skeleton, researchers imagined Torrejonia leaping through the trees withits skinny toes gripping onto the branches.
The result is that “Torrejonia is one of the oldest known primates, a distant cousin of ours. Researchers imagined that after these past approximate 60 million years of evolution these small proto-primates eventually adapted and evolved into bipedal-walking, philosophizing Apes, cousins of we Humans and another chapter in the mammals’ evolutionary journey, now 200 million years long and counting [Ascent of the Mammals by Stephen Brusatte and Zhe-Xi Luo
Scientific American 314(6):28-35 · May 2016 with 408 Reads DOI: 10.1038/scientificamerican0616-28
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“Palaechthonid plesiadapiforms from the Palaeocene of western North America have long been recognized as among the oldest and most primitive euarchontan mammals, a group that includes extant primates, colugos and treeshrews. Despite their relatively sparse fossil record, palaechthonids have played an important role in discussions surrounding adaptive scenarios for primate origins for nearly a half-century.
“Likewise, palaechthonids have been considered important for understanding relationships among plesiadapiforms, with members of the group proposed as plausible ancestors of Paromomyidae and Microsyopidae.
Stephen G. B. Chester, Thomas E. Williamson, Jonathan I. Bloch, Mary T. Silcox and Eric J. Sargis described in detail osteological, anatomical and potential physiological characteristics by analyzing the skeleton of Torrejonia wilsoni from the early Palaeocene (approx. 62 Ma) of New Mexico, which is the oldest known plesiadapiform skeleton and the first
postcranial elements recovered for a palaechthonid.
Results from a cladistic analysis that includes new data from this skeleton suggest that palaechthonids are a paraphyletic group of stem primates, and that T. wilsoni is most closely related to paromomyids.
“The postcranium of T. wilsoni indicates that it was similar to that of all other plesiadapiforms for which skeletons have been recovered in having distinct specializations consistent with arboreality.
{{{ “Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. The habitats pose numerous mechanical abilities to animals moving through them and lead to a variety of anatomical, behavioral and ecological consequences as well as variations throughout different species. [Cartmill, M. (1985). Climbing. In Functional Vertebrate Morphology, eds. M. Hildebrand D. M. Bramble K. F. Liem and D. B. Wake, pp. 73–88. Cambridge: Belknap Press] }}}
“Plesiadapiforms are a diverse group of euarchontan mammals known from the Palaeocene and Eocene of North America, Europe and Asia. This likely paraphyletic or even polyphyletic group of mammals [1,2] has long been considered primate-like based on aspects of their dental morphology, and the most recent phylogenetic analyses suggest that they are either stem primates [3–6
or stem members of Primatomorpha (Primates + Dermoptera) [7,8]. The fossil record suggests that primates and other placental mammals diversified following the Cretaceous–Paleogene boundary [9].
“The diversification of primates (sensu lato) was not instantaneous, as fossil-bearing strata from the Puercan North American Land Mammal ‘Age’ (NALMA) have only produced a small number of primitive plesiadapiform species (e.g. [10–12]), and the first occurrences of a wider diversity of plesiadapiform families (Palaechthonidae, Paromomyidae, Picrodontidae, Carpolestidae and Plesiadapidae) are not documented until the following Torrejonian NALMA [1,2].
“Although fragmentary, the dentitions of plesiadapiforms from the Torrejonian NALMA represent a variety of tooth forms that likely reflect a diversity of ecological specializations [13]. Postcranial fossils of early and middle Palaeocene plesiadapiforms (e.g. Palaechthonidae and Picrodontidae) are even more rare, so locomotor habits of virtually all of these taxa that predate the late Palaeocene are unknown. Alternatively, dentally associated partial skeletons from the late Palaeocene and early Eocene representing four plesiadapiform families (Plesiadapidae,
Carpolestidae, Paromomyidae and Micromomyidae) are known, and indicate that members of those families had a variety of positional behaviours, yet were all clearly arboreal [14]. More specifically, they likely frequently used orthograde (upright) postures while clinging and climbing on vertical supports and were capable of grasping small diameter branches with their hands and feet (e.g. [3,15–17]).
“However, these skeletons almost all represent relatively late occurring taxa that are well nested within their respective clades [14]. With the exception of several specimens of plesiadapids (e.g. [18]) and isolated tarsals attributed to Purgatorius [6], postcrania of early and middle Palaeocene plesiadapiforms, such as palaechthonids, have been lacking.
The early occurrence and hypothesized relatively basal position of the Palaechthonidae among plesiadapiforms suggests this family could be important for understanding euarchontan and early primate evolution, with some palaechthonids proposed to be ancestors of microsyopids [19] or paromomyids [2].
“Based on the only previously reported non-dental specimen of a palaechthonid, a partial cranium of Plesiolestes nacimienti, aspects of craniodental morphology (e.g. the presence of a large infraorbital foramen (IOF)) were cited as evidence that P. nacimienti was predominantly terrestrial, with adaptations similar to those of a hedgehog [20,21]. This inference was further extended to primitive primates more generally, implying that arboreality evolved later in primate evolution [21].
This hypothesis is assessed here by analysing the first postcrania known for the Palaechthonidae, a new partial skeleton of Torrejonia wilsoni (NMMNH P-54500), from the late Torrejonian (To3) of the Nacimiento Formation, San Juan Basin, New Mexico (figure 1).
Please review Torrejonia wilsoni skeleton details in the reference cited below.
This skeleton was found at locality NMMNH L-6898 in the Nacimiento Formation, San Juan Basin, New Mexico (collected under Bureau of Land Management excavation permit NM07-002E to T.E.W.). This locality is within the Torrejonian (To3) Mixodectes pungens zone of Torrejon Wash [22], which yields the type fauna for the Torrejonian NALMA [23,24]. The site is located within a normal polarity zone correlated with Chron C27n, which constrains the age of the site to about 62 Ma [25–27]. What is interpreted to be a partial skeleton of a single individual of T. wilsoni was recovered mixed with partial skeletons of two other mammals (see the electronic supplementary material), including a nearly complete skeleton of M. pungens (NMMNH P-54501) and a less complete skeleton of the eutherian mammal Acmeodon secans (Cimolestidae; NMMNH P-54499).
The right distal tibia, astragalus, calcaneus and cuboids of Torrejonia (figures 1 and 3) provide evidence for mobile ankle joints allowing increased inversion and eversion to adjust to uneven and variable arboreal supports. The upper ankle joint is represented by a distal tibia with a short medial malleolus and a dorsolaterally oriented and ungrooved articular surface that mirrors the lateral tibial facet of the astragalus. The extension of the astragalar lateral tibial facet onto the astragalar neck suggests a habitually dorsiflexed foot in Torrejonia, as is typical of mammals that cling to vertical supports such as tree trunks [34]. The calcaneal ectal facet is longer than the corresponding astragalar ectal facet allowing translation, and the calcaneal sustentacular facet is continuous distally onto the body, allowing increased inversion and eversion at the lower ankle joint [6,34]. The calcaneus is not distally elongated, in contrast to the condition in leaping euprimates. The peroneal process is very large, projects distolaterally, and has a groove on the lateral side for the tendon of peroneus longus, a muscle that would have contributed to eversion of the foot. The calcaneal cuboid facet is concave with a well-developed plantar pit, mirroring the proximal articular surface of the cuboids. Both features would have contributed to high degrees of inversion and eversion at the transverse tarsal joint.
Please see Figure 4. in reference: Hypothesis of evolutionary relationships of Torrejonia wilsoni and other eutherian mammals. (Left) Resulting single most parsimonious cladogram based on modified morphological dataset of Bloch et al. [4], sampling a total of 240 morphological characters (68 postcranial, 45 cranial and 127 dental) with Primates sensu lato indicated in blue and Torrejonia wilsoni supported as a stem primate and indicated in orange. Numbers below branches represent Absolute Bremer Support values. See the electronic supplementary material for detailed methods, descriptions of morphological characters, specimens examined (also see [5]), and the taxon-character matrix in TNT format. (Bottom) Simplified subset of resulting tree topology focused on Primates. Boxes (a–f) illustrate tarsals of select primates with great mobility at the upper ankle joint (yellow: lateral tibial facet extends distally onto neck of astragalus in dorsal view), lower ankle joint (red: sustentacular facet extends distally onto body of calcaneus in dorsal view) and transverse tarsal joint (orange: round, concave cuboid facet of calcaneus in distal view) indicating arboreality. Boxes (a–f) also illustrate micro X-ray CT scan reconstructions of (a) purgatoriid Purgatorius unio p4-m3 (UCMP 107406) with tall molar cusps in buccal view, (b) micromomyid Dryomomys szalayi cranium (UM 41870) in right lateral view with large IOF, (c) Torrejonia wilsoni partial skeleton (NMMNH P-54500), (d) paromomyid Ignacius graybullianus cranium (USNM 421608) in right lateral view with relatively large olfactory bulbs (OB) of endocast (violet), (e) carpolestid Carpolestes simpsoni cranium (USNM 482354) in right lateral view and tarsals (UM 101963) and (f) notharctid Notharctus tenebrosus cranium (AMNH 127167) in right lateral view. Some elements reversed for clarity. See figure 3 legend for specimen numbers of tarsals not listed above. See the electronic supplementary material for institutional abbreviations.
“Based on craniodental morphology, the palaechthonid P. nacimienti was reconstructed as a predominantly terrestrial early primate that relied heavily on tactile and olfactory information like a hedgehog or some other ground-dwelling insectivore [21].
“Dental features used to support this hypothesis include the relatively tall lower molar trigonid cusps of P. nacimienti, which closely resemble those of the oldest known plesiadapiform, Purgatorius, and suggest an omnivorous diet that included a large proportion of insects [20,21].
“Cranial features used to support this hypothesis include small, laterally oriented and widely separated orbits, and a large IOF that suggests the presence of many vibrissae and a well-innervated snout [21]. It was further proposed that primitive insectivorous and terrestrial primates such as P. nacimienti were similar to the last common ancestor of plesiadapiforms and euprimates, and that arboreality evolved in parallel in these two groups as they shifted to a more plant-based diet [21].
“The greatly improved plesiadapiform fossil record demonstrates that some of the cranial features previously cited as informative for a terrestrial substrate preference in P. nacimienti, such as relatively small and laterally oriented orbits and large infraorbital foramina, occur in other plesiadapiforms, such as the micromomyid Dryomomys, that have postcranial features indicative of arboreality (figure 4).
Furthermore, recent studies of endocranial anatomy provide additional evidence for limited visual processing and strong olfaction in arboreal plesiadapiforms (e.g. paromomyid Ignacius [39]). Also, isolated tarsal bones have recently been attributed to the geologically oldest plesiadapiform Purgatorius, and they suggest that fairly insectivorous primitive plesiadapiforms were arboreal [6]. These discoveries demonstrate that the suggested links between terrestriality and insectivorous dental traits, as well as terrestriality and cranial traits associated with less specialized vision, are not upheld in the early primate fossil record (figure 4).
Analysis of the partial skeleton of T. wilsoni demonstrates that this first palaechthonid known from postcrania was arboreal (tree inhabitat) and had capabilities for clinging and climbing on vertical supports like other plesiadapiforms.
Although arboreality in T. wilsoni is certainly not direct evidence for arboreality in P. nacimienti, the latter may have been arboreal given that it is phylogenetically bracketed by arboreal taxa such as T. wilsoni, paromomyids and micromomyids (figure 4) [3,14].
If it is discovered that P. nacimienti or any other taxon nested within Primates has features of the postcranium related to terrestriality, they would be secondarily derived. This implies that arboreality did not evolve in parallel in separate groups of primitive primates, but rather that primates are primitively arboreal (e.g. [3,6,15,34]) and plesiadapiforms represent an exclusively arboreal radiation based on all the relevant skeletal evidence, including the new partial skeleton of T. wilsoni analysed here.
[Oldest skeleton of a plesiadapiform provides additional evidence for an exclusively arboreal radiation of stem primates in the Palaeocene, by [Stephen G. B. Chester, Thomas E. Williamson, Jonathan I. Bloch, Mary T. Silcox and Eric J. Sargis, May 31, 2017 R. Soc. open sci. 4 170329 http://doi.org/10.1098/rsos.170329]
