Thank you for the excellent post. As people can see, the earliest branch within the group of primates is between the Lemur-like and Tarsier-like primates. The Lemur like one give rise to modern lemurs and lorises while the tarsier-like one branch more to give rise to monkeys/apes-humans. The evidence for this branch has accumulated recently.
One of the many reasons scientists believe that some kind of evolutionary method explains life is because the patterns found in the fossil record. Remains of animals found millions of years ago look very different than those living today. However as the fossils come from rocks laid down more closer to the present, many of the animals begin to resemble the forms biologists see today. This is true for the case of ourselves too.
In the earliest times after the extinction of the dinosaurs (65-55 million years ago), most of the primates that lived were quite different from modern varieties. But around 55 million years ago, fossils begin to turn up that look like ancestors to
modern day lemurs and lorises. These are called
Adapids, and the best preserved fossil is that of a species called Darwinius that lived 47 million years ago in Germany. It is an entirely complete skeleton, along with impression of fur and soft tissue, entombed in hardened shale.
https://en.wikipedia.org/wiki/Darwinius
Meanwhile a somewhat older fossil (55 million years), also encased in shell was found in China. This one, called
Archicebus, is an extremely small little primate and have features common to
modern day tarsiers and monkeys. It is considered to be one of the species of a closely related group that will eventually lead to tarsiers and monkeys/apes.
http://www.nature.com/news/oldest-pr...veiled-1.13142
Here we see that by 55-50 million years the ancient primates have divided into the two big groups into which all modern day primates belong. One group leads to lemurs and lorises and have species that look like Darwinius , and the other look a mix of tarsiers/monkeys of which Archicebus is a good example.
It is also important to note what is
not found. None of the modern day monkeys, lemurs, tarsiers, apes are found. What are found are quite distinct creatures that have traits in their body that resemble the modern day primates in some respects. The science of modern day cladistics identifies and quantifies all these traits and thus places these ancient creature in "trees" of resemblances between themselves and the modern primates. There will be many creatures in the millions of years afterwards that begin to resemble the modern creatures more closely until there comes a time (quite recent) when the actual modern forms crop up. This is a prediction of and good evidence for the theory of evolution in the living world.
Last time I posted fossil evidence from 50-55 million years ago that showed the point of divergence between lemur-like primates and tarsier-monkey-ape-like primates. For the next 20 million years these two broad divisions persisted and fossils of primates show no further distinction.
Then around
37-30 million year range, the monkey-ape like primates begin to diverge from their tarsier cousins.
Let me put in some shortcut name:-
Anthropoids:- The category to which New World Monkeys, Old World Monkeys and Apes Belong. The Anthropoids as a group has characters that distinguish it from other primates (like tarsiers or lemurs or lorises)
1) They have relatively smaller sized, forward facing eyes that look in the same direction. Basically a better binocular vision. This is seen in the orbital bone behind their eyes.
2) There is a bony plate behind their eyes that separate the eyes from their cheek muscles and bones (probably isolates them from vibration while chewing).
3) The bone structure in their middle and inner ear is also quite distinctive. The difference is probably to pick out peripheral noise, as the more convergent binocular vision makes the sides less covered relative to other primates.
4) There are also small but distinctive changes in their feet and heel bones.
Catarrhini:- Joint group of Old World Monkeys and Apes. These are the monkeys and apes that live in Asia and Africa.
New World Monkeys (Platyrrhini):- These are the monkeys that live in South America. Despite using the same confusing term "monkey" they are more distantly related (in genealogy as well as characteristics) to the monkeys of Asia-Africa than the apes are related to the monkeys of Asia-Africa. Its like calling Native Americans "Indians".
Difference between Old World Catarhinnis and the New World "monkeys".
1) A major difference is the teeth. While Tarsiers and ancient undifferentiated anthropoids has 3 premolars, the Old World Monkey-Apes lost one premolar to have only two on each half row. So the dental formula of Old World Monkeys-Apes are 2.1.2.3 instead of 2.1.3.3 for the new world monkeys.
2) The nose of Catarhinnis face downwards while that of New World Monkeys face sideways. That is the term Catarhinni means :- "flat nosed". This create distinction in the nasal cavity in skulls as well.
There are also smaller differences in ear bones, skull shapes etc. that act as distinguishing the Old World and New World groups
http://www.cabrillo.edu/~crsmith/monkeycomparisons.html
There is a place in Egypt called the Fayum depression which preserved an astonishingly detailed record of fossil animals and plants that lived in (the then forested) North Africa 37-30 million years ago. Some of the best preserved Anthropoids have been found from this site. This one site has preserved remains of over 20 different primates that straddle the divergence between and within the Anthropoids.
For example we have the fossil of
Parapithecus which is a small primate belonging to the Anthropoid group just as they are differentiating from tarsiers and before they have diverged between New World and Old World types. Its a tiny little creature possessing most of the basic anthropoid character but none of the special traits of the New/Old lineages. Note how small it is.
Paper
The same place also provided the remains of the most primitive Catarhinni (Old World monkey-ape).
Catopithecus is one such species having the distinct Catarhinni dental formula of 2 premolars instead of three. It has several characteristics of Catarhinni, but many other features are still of the more undifferentiated anthropoid types. So it is almost near the divergence point between the New World and Old World groups of primates.
http://fossils.valdosta.edu/fossil_pages/fossils_ter/p33.html
The same Fayuum sequence also revealed the first fully developed Catarhinni (Old World Ape-Monkey), a species called
Aegyptopithecus. Varying in weight from 2-4 kg its quite large among the primates of its time and has all the features of a Catarhinni. Dubbed as the dawn-ape, a species like this is considered the basic ancestral "stock" from which Old World Monkeys and Apes came from. Something that is or is quite close to the species groups that are the common ancestors of all Afro_Asiatic monkeys and apes.
Here is an excellent article on what is known about this "dawn ape". Notably while it had good vision, it had quite a small brain compared to modern monkeys or apes.
http://www.livescience.com/1526-human-ancestor-pea-brain.html
What about the New World monkeys? Amazingly, fossils of groups that either were ancestors to "monkeys" of South America or quite closely related to such ancestors turn up in the Fayum depression site as well! This will be the
Proteopithecus. They show such a mosaic of New World, general anthropoid and some Old World features that most researchers place them at the very divergence point between the Old and New World groups..maybe slightly in the New World (side-nosed) side of the spectra.
The scientific article analyzing its skull says
ABSTRACT Recent discovery of crania, dentitions, and postcrania of a primitive anthropoidean primate, Proteopithecus sylviae, at the late Eocene L-4l quarry in the Fayum, Egypt, provides evidence of a new taxonomic family of early African higher primates, the Proteopithecidae. This family could be part of the basal radiation that produced the New World platyrrhine primates, or it could be unrelated to any subsequent lineages. Although no larger than a small callitrichid or a dwarf lemur, this tiny primate already possessed many of the derived features of later anthropoids and was a diurnal and probably dimorphic species. In dental formula and other dental proportions, as well as in known postcranial features, Proteopithecus more nearly resembles platyrrhines than does any other Old World higher primate. The small size of the Proteopithecus cranium demonstrates that the defining cranial characteristics of Anthropoidea did not arise as a consequence of an increase in size during derivation from earlier prosimians.
http://www.pnas.org/content/94/26/14970.full.pdf
What is the summary? One sees that in Africa, between 37-30 million years ago, there lived and evolved many kinds of primates that can be placed as basic anthropoids, primates at the divergence point of New World and Old World types and the first Old World monkey-ape type species that are likely to be the ones from which all current Old World moneys and apes came from. Fayum depression and other places show evidence of the branching of the monkey-apes from tarsiers and their eventual further branching into Old world and New World forms during this crucial times. Its hard to see what other than evolution through descent could explain such patterns. Note the pattern. Lemur-like and tarsier-like primates are seen to diverge at 55 mya and they look the furthest apart today. Within the tarsier-like, some groups begin to become more monkey-ape like and diverge themselves into New and Old World monkey-ape types. This is seen to occur later, between 40-30 million years ago (mya), just as evolution would predict. The prediction of sequential, staggered in time, divergence based in current similarity-dissimilarity is a prediction that is confirmed by the past fossil-sequence.
