Transitional fossil: Difference between revisions

Revision as of 14:40, 13 October 2021

Revision as of 08:32, 14 November 2021

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The hominid ''Australopithecus afarensis'' represents an evolutionary transition between modern bipedal humans and their quadrupedal [[ape]] ancestors. A number of traits of the ''A. afarensis'' skeleton strongly reflect bipedalism, to the extent that some researchers have suggested that bipedality evolved long before ''A. afarensis''.<ref name="Lovejoy1988">{{cite journal |last=Lovejoy |first=C. Owen |author-link=Owen Lovejoy (anthropologist) |date=November 1988 |title=Evolution of Human walking |url=http://users.clas.ufl.edu/krigbaum/proseminar/Lovejoy_1988_SA.pdf |journal=[[Scientific American]] |volume=259 |issue=5 |pages=82–89 |doi=10.1038/scientificamerican1188-118 |issn=0036-8733 |pmid=3212438 |bibcode=1988SciAm.259e.118L }}</ref> In overall anatomy, the pelvis is far more human-like than ape-like. The [[Ilium (bone)|iliac blades]] are short and wide, the sacrum is wide and positioned directly behind the hip joint, and there is clear evidence of a strong attachment for the [[Rectus femoris muscle|knee extensors]], implying an upright posture.<ref name="Lovejoy1988" />{{rp|122}}

While the [[pelvis]] is not entirely like that of a human (being markedly wide, or flared, with laterally orientated iliac blades), these features point to a structure radically remodelled to accommodate a significant degree of [[bipedalism]]. The [[femur]] angles in toward the knee from the [[hip]]. This trait allows the foot to fall closer to the midline of the body, and strongly indicates habitual bipedal locomotion. Present-day humans, [[orangutan]]s and [[spider monkey]]s possess this same feature. The feet feature [[Adducted|adducted]] big toes, making it difficult if not impossible to grasp branches with the [[hindlimb]]s. Besides locomotion, ''A. afarensis'' also had a slightly larger brain than a modern [[Common chimpanzee|chimpanzee]]<ref name="SmithsonianHuman">{{cite web |url=http://humanorigins.si.edu/evidence/human-fossils/species/australopithecus-afarensis |title=''Australopithecus afarensis'' |website=Human Evolution |publisher=[[Smithsonian Institution]]'s Human Origins Program |___location=Washington, D.C. |access-date=2015-05-15}}</ref> (the closest living relative of humans) and had teeth that were more human than ape-like.<ref>{{cite journal |last1=White |first1=Tim D. |author-link1=Tim D. White |last2=Suwa |first2=Gen |last3=Simpson |first3=Scott |last4=Asfaw |first4=Berhane |author-link4=Berhane Asfaw |date=January 2000 |title=Jaws and teeth of ''Australopithecus afarensis'' from Maka, Middle Awash, Ethiopia |journal=[[American Journal of Physical Anthropology]] |volume=111 |issue=1 |pages=45–68 |doi=10.1002/(SICI)1096-8644(200001)111:1<45::AID-AJPA4>3.0.CO;2-I |issn=0002-9483 |pmid=10618588}}</ref>

===Pakicetids, ''Ambulocetus''===

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''Tiktaalik'' is a genus of extinct [[Sarcopterygii|sarcopterygian]] (lobe-finned fish) from the Late [[Devonian]] period, with many features akin to those of tetrapods (four-legged animals).<ref name="Nature">{{cite journal |last1=Daeschler |first1=Edward B. |author-link1=Ted Daeschler |last2=Shubin |first2=Neil H. |author-link2=Neil Shubin |last3=Jenkins |first3=Farish A., Jr. |author-link3=Farish Jenkins |date=6 April 2006 |title=A Devonian tetrapod-like fish and the evolution of the tetrapod body plan |journal=Nature |volume=440 |issue=7085 |pages=757–763 |bibcode=2006Natur.440..757D |doi=10.1038/nature04639 |issn=0028-0836 |pmid=16598249 |doi-access=free }}</ref> It is one of several lines of ancient sarcopterygians to develop adaptations to the oxygen-poor shallow water habitats of its time—adaptations that led to the evolution of tetrapods.<ref name="scientificamerican">{{cite journal |last=Clack |first=Jennifer A. |author-link=Jenny Clack |date=December 2005 |title=Getting a Leg Up on Land |journal=Scientific American |volume=293 |pages=100–107 |issue=6 |bibcode=2005SciAm.293f.100C |doi=10.1038/scientificamerican1205-100 |issn=0036-8733 |pmid=16323697 }}</ref> Well-preserved fossils were found in 2004 on [[Ellesmere Island]] in [[Nunavut]], Canada.<ref>{{cite journal |last=Easton |first=John |date=23 October 2008 |title=''Tiktaalik's'' internal anatomy explains evolutionary shift from water to land |url=http://chronicle.uchicago.edu/081023/tiktaalik.shtml |journal=University of Chicago Chronicle |issue=3 |volume=28 |issn=1095-1237 |access-date=2012-04-19 }}</ref>

''Tiktaalik'' lived approximately 375&nbsp;million years ago. [[Paleontology|Paleontologist]]s suggest that it is representative of the transition between non-tetrapod vertebrates such as ''[[Panderichthys]]'', known from fossils 380&nbsp;million years old, and early tetrapods such as ''[[Acanthostega]]'' and ''[[Ichthyostega]]'', known from fossils about 365&nbsp;million years old. Its mixture of primitive fish and derived tetrapod characteristics led one of its discoverers, [[Neil Shubin]], to characterize ''Tiktaalik'' as a "[[Tetrapodomorpha|fishapod]]."<ref>{{cite news |last=Wilford |first=John Noble |author-link=John Noble Wilford |date=5 April 2006 |title=Scientists Call Fish Fossil the 'Missing Link' |url=https://www.nytimes.com/2006/04/05/science/05cnd-fossil.html |newspaper=[[The New York Times]] |access-date=2015-05-17}}</ref><ref>{{harvnb|Shubin|2008}}</ref> Unlike many previous, more fish-like transitional fossils, the "fins" of ''Tiktaalik'' have basic wrist bones and simple rays reminiscent of fingers. They may have been [[weight-bearing]]. Like all modern tetrapods, it had rib bones, a mobile neck with a separate pectoral girdle, and lungs, though it had the gills, scales, and fins of a fish.<ref name="Nature" /> However in a 2008 paper by Boisvert at al. it is noted that ''Panderichthys'', due to its more derived distal portion, might be closer to tetrapods than ''Tiktaalik'', which might have independently developed similarities to tetrapods by convergent evolution.<ref>{{cite web|url=http://uu.diva-portal.org/smash/get/diva2:221262/FULLTEXT01 |title=Pectoral fin info |publisher=uu.diva-portal.org |date= |accessdateaccess-date=2021-05-09}}</ref>

Tetrapod footprints found in Poland and reported in ''[[Nature (journal)|Nature]]'' in January 2010 were "securely dated" at 10&nbsp;million years older than the oldest known [[Elpistostegalia|elpistostegids]]<ref>{{cite journal |last1=Niedźwiedzki |first1=Grzegorz |last2=Szrek |first2=Piotr |last3=Narkiewicz |first3=Katarzyna |last4=Narkiewicz |first4=Marek |last5=Ahlberg |first5=Per E. |author-link5=Per E. Ahlberg |date=7 January 2010 |title=Tetrapod trackways from the early Middle Devonian period of Poland |journal=Nature |volume=463 |issue=7227 |pages=43–48 |doi=10.1038/nature08623 |issn=0028-0836 |pmid=20054388 |display-authors=3 |bibcode=2010Natur.463...43N |s2cid=4428903 }}</ref> (of which ''Tiktaalik'' is an example), implying that animals like ''Tiktaalik'', possessing features that evolved around 400&nbsp;million years ago, were "late-surviving relics rather than direct transitional forms, and they highlight just how little we know of the earliest history of land vertebrates."<ref>{{cite journal |author=<!--Staff writer(s); no by-line.--> |date=7 January 2010 |title=Four feet in the past: trackways pre-date earliest body fossils |url=http://www.nature.com/nature/journal/v463/n7277/edsumm/e100107-01.html |type=Editor's summary |journal=Nature |volume=463 |issue=7227 |issn=0028-0836}}</ref>

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{{Main|Runcaria|Evolution of plants#Seeds}}

A Middle Devonian precursor to [[Spermatophyte|seed plant]]s has been identified from Belgium, predating the earliest seed plants by about 20&nbsp;million years. ''Runcaria'', small and radially symmetrical, is an integumented [[Sporangium|megasporangium]] surrounded by a [[Calybium and cupule|cupule]]. The megasporangium bears an unopened [[Distal|distal]] extension protruding above the multilobed [[Integument#Botanical usage|integument]]. It is suspected that the extension was involved in [[Anemophily|anemophilous pollination]]. ''Runcaria'' sheds new light on the sequence of character acquisition leading to the seed, having all the qualities of seed plants except for a solid [[seed coat]] and a system to guide the pollen to the seed.<ref>{{cite journal |last1=Gerrienne |first1=Philippe |last2=Meyer-Berthaud |first2=Brigitte |last3=Fairon-Demaret |first3=Muriel |last4=Streel |first4=Maurice |last5=Steemans |first5=Philippe |date=29 October 2004 |title=Runcaria, a Middle Devonian Seed Plant Precursor |journal=[[Science (journal)|Science]] |volume=306 |issue=5697 |pages=856–858 |bibcode=2004Sci...306..856G |doi=10.1126/science.1102491 |issn=0036-8075 |pmid=15514154 |s2cid=34269432 |display-authors=3 }}</ref>

==Fossil record==

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*{{cite web |url=http://www.talkorigins.org/faqs/patterson.html |title=Patterson Misquoted: A Tale of Two 'Cites' |last=Theunissen |first=Lionel |date=24 June 1997 |website=[[TalkOrigins Archive]] |publisher=The TalkOrigins Foundation, Inc. |___location=Houston, TX |access-date=2015-05-19}}</ref> This theory, however, pertains only to well-documented transitions within taxa or between closely related taxa over a geologically short period of time. These transitions, usually traceable in the same geological outcrop, often show small jumps in morphology between extended periods of morphological stability. To explain these jumps, Gould and Eldredge envisaged comparatively long periods of genetic stability separated by periods of rapid evolution. Gould made the following observation concerning creationist misuse of his work to deny the existence of transitional fossils:

{{quoteblockquote|Since we proposed punctuated equilibria to explain trends, it is infuriating to be quoted again and again by [[Creationism|creationists]]—whether through design or stupidity, I do not know—as admitting that the fossil record includes no transitional forms. The punctuations occur at the level of species; directional trends (on the staircase model) are rife at the higher level of transitions within major groups. | [[Stephen Jay Gould]] |''[[The Panda's Thumb (book)|The Panda's Thumb]]''{{sfn|Gould|1980|p=189}}}}

==See also==