8.I: Introduction

This chapter will address many questions that may never have occurred to you.  What are the advantages of warm-bloodedness?  Why do we see and hear better than we smell?  Why do we love our mothers?  Why do we care so much about socializing, status, and gossip?  These are all issues that predate human nature.  They come from our nature as mammals and primates

The timeline below shows the three geological eras of the last few hundred million years.  Each era is divided into periods.  This chapter mostly belongs to the Mesozoic or “Middle Life” era, the time of the first mammals as well as dinosaurs.  The animals below the timeline are close proxies for our ancestors.     

Section II is a global overview of the Mesozoic Era.  This era was dramatically bookended by two major mass extinctions.  The mass extinction that ended the Paleozoic era, called the Permian-Triassic or simply the P-T event, was the most severe mass extinction in world history.  The Cretaceous-Paleogene or K-Pg event is even more famous because it was the demise of the dinosaurs.  Both of these mass extinctions had significant influence on the evolution of our ancestors.  The P-T event was an evolutionary bottleneck, narrowing down the gene pool and leaving only a lucky minority of species to repopulate the Earth. 

Dinosaurs’ disappearance cleared the way for mammals to flourish in a variety of niches.  Section III takes a closer look at mammals, our biological class.  By the end of this time scale, our ancestors were primates who were already beginning to show signs of social and neurological sophistication.  Although they still didn’t look much like us, at least they were becoming cute, cuddly, and cunning.

See below for image attributions.


Continue to Section 8.II: The Mesozoic World


Volcano image by Pixabay user DoomSlayer, Public Domain, https://pixabay.com/vectors/magma-danger-mountain-hot-lava-5267956/ (accessed and saved 8/02/20).

Meteor image by Pixabay user “monhtm”, Public Domain, https://www.needpix.com/photo/download/640409 (accessed and saved 8/02/20).

Dinosaur icon public domain (accessed and saved 11/22/20).

Early amniote (“reptile”) image by User:ArthurWeasley, GFDL / CC BY-SA 3.0 license, https://commons.wikimedia.org/wiki/File:Archaeothyris_BW.jpg (accessed and saved 8/24/19). 

Early mammal image “Morganucodon” by FunkMonk (Michael B. H.), CC BY-SA 3.0 license, https://commons.wikimedia.org/wiki/File:Morganucodon.jpg (accessed, saved, and archived 11/22/20).

Early catarrhine (“monkey”) image by Nobu Tamura, CC BY-SA 3.0 license, https://commons.wikimedia.org/wiki/File:Aegyptopithecus_NT.jpg (accessed and saved 8/24/19).

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