Post on 13-May-2023
EARLY PRIMATE EVOLUTIONEARLY PRIMATE EVOLUTIONWhy did the primates appear?Who were their ancestors?Why did primates diversify?
How genes in living primates can help us answer these questions?
How the closest living relatives of primates can help us answer these
questions?
Early Early Primate Primate EvolutiEvoluti
onon•Ancestor of all primates unknown;
•Molecular data indicate an ancient split between strepsirhines and haplorhines probably before 65 Ma
Climate Change and Primate Climate Change and Primate Origins and EvolutionOrigins and Evolution
•How has climate change affected primate evolution over time?
•What factors drive climate change?
Cenozoic Climates: From Greenhouse from Cenozoic Climates: From Greenhouse from IcehouseIcehouse• Past climate change studies shed light on the natural selection factors
which drove primate evolution• During the Cretaceous period (130-65 million years ago), the Earth was
unusually warm (mean temperature = 10 to 15 ºC warmer than today).
Forests spread to latitudes that today are tundra or ice covered. No polar ice caps.
Long-term climatic trends
revealed by oxygen
isotopesOverall cooling since the onset of the Eocene 55 mya.
Foraminifera
Cenozoic climates• Cenozoic period = the age
of mammals: 65 Ma – present
• Appearance of the first mammals, and primates, of modern aspect
• Early Cenozoic global climate = warm, wet and non-seasonal with little difference between the equators and the poles
• No glaciers and ice sheets• High precipitation
forests were the dominant type of vegetation, reached the Arctic and Antarctic
Cenozoic climatesEpochs within the Cenozoic:• Paleocene (65-56 Ma) • Eocene (55-34 Ma) • Oligocene (34-23 Ma)• Miocene (23 – 5 Ma)• Pliocene (5-2.5 Ma)• Pleistocene (2.5 Ma – 10 000 BC) =
The “Ice ages”• Holocene (10 000 BC – present)
World temperatures rose slightly during the Paleocene to peak in the early Eocene
Short-term increase followed by a long-term cooling including at least 5 abrupt downturns:
34 Ma early Oligocene23 Ma terminal Oligocene/early Miocene14 Ma mid-Miocene6-5 Ma late Miocene3.1-2.8 Ma mid-Pliocene
Key tectonic events in the Cenozoic – relevance to climate:
Events which led to continental glaciation in Antarctica and the northern hemisphere:
• During the Cretaceous and early Cenozoic, Antarctica largely ice-free;
• Still connected or at least close to South America and Australia.
Warm ocean currents, deflected by South America and Australia bathed the coast of Antarctica.
• By around 35 million years ago, the last connections with Australia and South America were severed.
Deep water passages surrounded Antarctica formation of a continuous Circum- Antarctic cold current that thermally isolated Antarctica.
Cenozoic climatesCooling events associated with: Continental drift likely played a key role in global
cooling: - redirect oceanic currents, which transfer heat
between latitudes - continental collision results in mountain
building, which alters atmospheric circulation and lead to a decrease in CO2
Cenozoic climates As a result of continental drift, the climate changed
from warm and wet to increasingly cooler and drier. Forests replaced by open savannahs and grasslands.
• How about the Ice Ages? (initiated 2.5 Ma):How about the Ice Ages? (initiated 2.5 Ma): The mechanism that drove late Cenozoic cold-warm
(glacial-interglacial) rapid and intense alternation during the Ice ages:
interaction between astronomical variables, and oceanic and atmospheric circulation•Astronomical variables shown to
influence global climate on Earth are:
•The shape of the earth’s orbit – from elliptical to round (E)
•The tilt of the spin axis (T)
•The wobble of the earth’s spin axis (P)
Ecological Context for Primate origins• The Cretaceous witnessed the
appearance of flowering plants, which became dominant;
• Their success created niches for animals feeding on nectar, flowers, leaf buds, and fruits;
Explosion of insectsBirds and mammals diversified to
exploit the increase of edible plant parts and the insects;
Diversification of placental mammals;
appearance of modern-day orders
• Earliest placental mammals (~ 130 Ma) had diverse adaptations, but all were more or less similar to shrews
• The earliest primates continued to eat insects, but started developing molars with less pointed cusps and blunter ridges started eating fruits and other vegetal matter
Ecological Context of Primate origins
Paleocene Paleocene mammalsmammalsPrimitive
creatures from which the primates sprung surely appeared between 80 to 65 Ma
Plesiadapiformes - the primate ancestors?• Dinosaur extinction opened
ecological opportunities for birds and mammals who burgeoned between 65 and 55 Ma
• One such branch were the Plesiadapiformes, regarded as close to the line producing primates
• Plesoadapiformes known from western North America and western Europe, in subtropical climate
• Small (shrew to cat size), omnivorous
• Some concentrated more on insects, while other more on seeds or fruits
Plesiadapiformes - the primate ancestors?
• Primitive skulls: - large snouts with
laterally-placed eyes (well-developed sense of smell and limited overlap of the fields of vision)
- eye sockets open to the sides rather than surrounded by postorbital bars.
• In most species, the pointed terminal phalanges indicate that all digits retained claws: no grasping hands and feet
Exception: genus Carpolestes at 55 Ma: big toe had a nail and could have been opposable
Plesiadapiformes - the primate ancestors?
-- No forward-facing eyes-- tiny – 100 g, 35 cm long with the tail-- Evolved to exploit the diversity of fruits, flowers, leaf buds increased in the Paleocene took to the trees to exploit this new food source and avoid competition with rodents.
• Plesiadapiformes linked to primates by: - shape of their molars and premolars - by possible presence of auditory bulla
Plesiadapiformes - primate ancestors?
• Yet, Plesiadapiformes resemble living tropical squirrels: - anterior teeth too specialized (large procumbent
incisors, reduced number of teeth) - cannot have given the generalized, primitive incisors and
canines and the large number of premolars of the first primates of modern
aspect. Plesiadapiformes likely a side branch, closely related to
primates, but not directly ancestral
Plesiadapiformes - primate ancestors?
Plesiadapiformes: likely side branch
Didn’t give rise toprimates
Gave adapids
and modern lemurs/lor
ises
Two hypotheses about the fate of the Plesiadapiformes
The First Primates of modern Aspect: Euprimates
What happened to the Plesiadapiformes if they did not give rise to the Primates?
became extinct in the late Paleocene or the early Eocene (56 - 54 Ma), likely because of unsuccessful competition with evolving primates of modern aspect.
The earliest fossils to be widely recognized as primates of modern aspect – jaws and teeth dated at 55 Ma.
Exhibit the suite of derived primate characteristics: - auditory bulla, - postorbital bar, - orbital convergence, - large brains relative to body size, - expanded visual areas of the brain - reduced olfactory bulbs, - grasping hands and feet, and nails on most if not all
digits.
The First Primates of Modern Aspect: Euprimates• Early Eocene: geographically and climatically similar to
the Paleocene• North America and western Europe connected by forested
land bridge, with subtropical climate• Early true primates (Euprimates) widely dispersed, and
similar between the two continents• Continental drift disrupted land connection between
Europe and N. America ~ 50 Ma
Distribution and Classification of the Euprimates• In the Eocene (55
to 34 Ma):• Very abundant Euro-American
fossils commonly divided between two groups: more lemur- like and more tarsier-like
• Both groups existed at the same time in southern and eastern Asia, northern Africa, and even India
Two diverse groups:
• Adapiformes, lemur-like (with large snouts), basal to the Strepsirhini (more closely related to Strepsirhini than to Haplorhini yet may not be directly ancestral)
• Omomyiformes, tarsier-like, basal to the Haplorhini (more closely related to Haplorhini than to Strepsirhini, yet may not be directly ancestral)
• Adapiformes were larger than omomyiformes, with body plan similar to larger extant lemurs
Distribution and Classification of Distribution and Classification of the Euprimatesthe Euprimates
Adapiformes had small orbit size: diurnal;
Omomyiformes had large orbital size: nocturnal
Euprimates: Adapiformes and Euprimates: Adapiformes and OmomyiformesOmomyiformes• Diet: most species in both groups focused on fruits,
although the really small ones ate insects while the larger ones focused on leaves
Euprimates: Adapiformes and Euprimates: Adapiformes and OmomyiformesOmomyiformes• Locomotion: in both groups:
- some hindlimb-dominated active arboreal quadrupeds like lemurs
- some slow arboreal quadrupeds like lorises
- some with elongated feet indicative of galago or tarsier-like ability to leap
• Adapiformes shared unique derived ear structure with lemurs and lorises (Ear ring), while omomyiformes – with tarsiers (Ear tube)
• Omomyiformes had more globular brains with more developed visual areas and smaller olfactory bulbs
• Adapiformes and Omomyiformes differed from living forms by being more primitive:
Adapiformes relative to lemurs/lorises: - 4 premolars on each side of the jaw (instead
of 2), - no tooth comb, - fused two halves of the lower jaw, like modern
haplorhines! - might have had sexually dimorphic canines as
anthropoids!
• Adapiformes and Omomyiformes differed from living forms by being more primitive:
Omomyiformes relative to tarsiers: - had a rhinarium (indicated by large gap between
the upper incisors)
Anthropoid OriginsAnthropoid OriginsOn anatomical grounds, which group is closer to the ancestry of
anthropoids (monkeys and apes)? Contentious!
Omomyiform teeth and skulls similar to modern anthropoid ones in some respects (smaller snouts and larger brains), but adapiformes share features with EARLY anthropoids that neither share with omomyiforms, such as:
- relatively long snout
- fusion of the two halves of the mandible in the midline in several
adapiform species and all anthropoids but no omomyiformes
- short, vertical and wide incisors versus more procumbent and sharply-
pointed ones in omomyiformes
- large, sexually-dimorphic canines in some adapiformes and in anthropoids
- lack of external auditory tube in adapiformes and early anthropoids
(presence in omomyiformes)
Anthropoid OriginsAnthropoid Origins Impossibility to distinguish between derived
similarity due to common descent (synapomorphy) and similarities that evolved in parallel (convergence) due to common adaptation
BUT, IF the tarsiers evolved from omomyiformes and IF tarsiers and anthropoids are sister taxa (more closely related to each other than either
is to lemurs/lorises)
Then probably omomyiformes were closer to the anthropoid ancestry
Then similarities between adapiforms and early anthropoids were convergences.
Theories of Primate OriginsTheories of Primate Origins• Why did the Primate pattern appear?1)Arboreal theory: the primate pattern resulted from natural selection
for features that permitted life in the trees: - collarbones providing stability of shoulder, - increased mobility of arm, - grasping hands and feet – good manipulative ability. - stereoscopic (3D) vision, - good eye to hand coordination
• Arboreal theory is criticized non-primate arboreal animals such as opossums, tree-
shrews and squirrels do not possess these characteristics;
2) Visual/Predation Hypothesis (“bug-snatching” theory):
• Grasping and visual abilities of the first primates were selected for seeing and catching insects in the understory of tropical forests.
Why did the Primate pattern Why did the Primate pattern appear?appear?
3) Extension and Update of the Arboreal theory: “Angiosperm radiation hypothesis”:
• The appearance and spread of flowering plants paved the way for a shift from an insectivorous diet to reliance on edible plant parts In the primate ancestors
• Natural selection enhanced the efficiency for foraging and harvesting flowering plants.
• Evolution in the locomotor, grasping, manipulative, cognitive, visual abilities
• Primitive primates had to : remember sites where patches of fruits are available,
when they will be ripe,
distinguish them from leaves, be able to reach them and snatch them and to remove any
inedible tissues.
Why did the Primate pattern Why did the Primate pattern appear?appear?
The Closest Living Relatives of The Closest Living Relatives of PrimatesPrimates The tree shrews (order Scandentia)• a variety of species living in the forests of Southern
Asia• Squirrel-sized, highly active, diurnal, largely
terrestrial living in the forest undergrowth• Long body, tail and muzzle, eyes on the side of the
head, claws
• One genus is arboreal and nocturnal - eats insects, vegetal material and small vertebrates
The Closest Living Relatives of The Closest Living Relatives of PrimatesPrimates•Primate-like characters of tree shrews:
- postorbital bar - enlarged visual areas of the brain - partial mobility of the big toe and thumb•Dermoptera (flying lemurs) might be more closely related to Primates than Scandentia
Flying lemurs
Inferring phylogenetic relationships and split times from the genes of living taxa• DNA sequences for phylogenetic reconstruction are powerful:
unlike anatomical differences, the likelihood that the nucleotide was changed with exactly the same other nucleotide and at exactly the same spot in the huge genome in two separate species or even individuals is minuscule.
This advantage is used in the technique known as
“ Molecular Clock”
Inferring phylogenetic relationships and split times from the genes of living taxaMolecular clock: • Allows determining the split patterns in an evolutionary tree, and even associated split dates
• Assumes that mutations in certain non-functional DNA regions have occurred randomly and at constant rates over long periods of time
Accumulation of more differences between two species implies a more ancient split, than would be the case if they differ by fewer mutations