8.V: Human Ancestor Gallery

These particular specimens are selected as basal embodiments of their clades, so they are known to be pretty closely related to our line of descent. Dates are grossly rounded.

300 MYASynapsids are the amniotes that are more closely related to today’s mammals than to reptiles or birds. The marker that traces synapsid lineage is an internal feature, the pattern of holes in our skull.  The first synapsids were still reptile-like. 1


Cynodonts came about halfway through the reptile-mammal transition.  The early cynodonts were the ancestors who lived through the worst of the P-T Extinction while almost all other tetrapods died around them.  They had noticeably more erect legs than earlier synapsids, and their transition to warm-bloodedness was probably mostly complete. 23


200 MYA:  It is difficult to know for sure, but fur may have evolved in pre-mammalian cynodonts. 4


Mammal skeletal features are recognizable in fossils dating back a little further than the 200-million-year mark.  Mesozoic mammals were tiny, and they evolved slowly. 5


150 MYA:  Eutheria was a transitional form.  The earliest eutheria still laid eggs, but their descendants all give live birth today. 6


70 MYA:  Placental mammals give live birth.  The earliest placental species has been modeled in an amazing supercomputer project that analyzes fossils and reconstructs their common ancestors. 7 This hypothetical critter, the Eve of all wombs, has been named Shrewdinger by popular vote.  Her “official” image is copyrighted, so I can’t duplicate it here, but the story is too cool to pass up.  Here’s the link to that picture and a video describing the project.  Shrewdinger basically looked like a shrew: 8


70 MYA:  Primates emerged as part of the early Cenozoic mammal radiation.  We are defined mostly by our grasping hands and forward-looking eyes. 9


50 MYA:  Simians are active during the day.  They have larger brains and more advanced social structures than earlier primates.  Catarrhines in particular have opposable thumbs and human-like teeth, but for the long canines.  The tail started to lose its function as a useful limb and gradually became weaker and shorter. 10


Back to Section 8.IV: Chapter 8 Summary


Chapter 8 Margin Notes (Blog Posts)


  1. Synapsid image by User:ArthurWeasley (Own work), GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/), https://commons.wikimedia.org/wiki/File:Archaeothyris_BW.jpg (accessed and saved 8/24/19).
  2. T.S. Kemp, Mammal-like Reptiles and the Origin of Mammals, Academic Press (1982; 2nd printing, New York, 1984) p. 301.
  3. Cynodont image by Nobu Tamura (http://spinops.blogspot.com) (Own work), GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/), https://commons.wikimedia.org/wiki/File:Procynosuchus_BW.jpg (accessed and saved 8/24/19).
  4. Furry cynodont image by Smokeybjb (Own work), CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0), https://commons.wikimedia.org/wiki/File:Brasilitherium_riograndensis.jpg (accessed and saved 8/24/19).
  5. Basal mammal photograph by Nordelch (Megazostrodon Natural History Museum), CC BY-SA 2.0 (http://creativecommons.org/licenses/by-sa/2.0), https://commons.wikimedia.org/wiki/File:Megazostrodon.jpg (accessed and saved 8/24/19).
  6. Eutheria image by Nobu Tamura http://paleoexhibit.blogspot.com/ http://spinops.blogspot.com/ http://www.palaeocritti.com (Own work), CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0), https://commons.wikimedia.org/wiki/File:Juramaia_NT.jpg (accessed and saved 8/24/19).
  7. Maureen A. O’Leary et al., “The placental mammal ancestor and the post-K-Pg radiation of placentals”, Science 339(6120):662-667 (2/08/2013), https://www.ncbi.nlm.nih.gov/pubmed/23393258 (accessed and saved 8/24/19).
  8. Shrew image public domain, https://www.wpclipart.com/animals/S/shrew/Large_Tree_shrew__Tupaia_tana.png.html (accessed and saved 8/24/19).
  9. Primate image by Mat Severson (Own work), CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0), https://commons.wikimedia.org/wiki/File:Reconstruction_image_of_Archicebus.jpg (accessed and saved 8/24/19).
  10. Catarrhine image by Nobu Tamura (http://spinops.blogspot.com) (Own work), CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0), https://commons.wikimedia.org/wiki/File:Aegyptopithecus_NT.jpg (accessed and saved 8/24/19).
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