Chapter 6

Macroevolution and the Early Primates

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What Is Macroevolution?

When and Where Did the First Primates Appear, and What Were They Like?

When Did the First Monkeys and Apes Appear, and What Were They Like?

Macroevolution and the Process of Speciation

macroevolution = evolution above the species level (i.e. the creation of new species)

speciation = the process of forming new species.

Macroevolution and the Process of Speciation

Since species are populations who are capable of interbreeding and producing viable and fertile offspring

To create new species, populations must separate through reproductive isolating mechanisms – biological and behavioral factors that separate breeding populations and prevent gene flow

Reproductive Isolating Mechanisms: For Class Discussion

What types of biological (morphology, genetics) and behavioral factors could lead to reproductive isolation?

Macroevolution and the Process of Speciation

Cladogenesis = speciation occurs when descendants of an ancestral population become reproductively isolated

Anagenesis = speciation through the gradual accumulation of advantageous traits

The Shape of Macroevolution

(A) cladogenesis; (B) convergent evolution; (C) parallel evolution

Cladogenesis

(Divergent Evolution)

Convergent Evolution

Ancestral populations are very distantly related

Convergent Evolution

Ancestral populations are very distantly related

Parallel Evolution

Ancestral populations are more closely related

The Timing of Evolution

Gradual evolution – suggested by Charles Darwin, physical traits change slowly and consistently over time

Punctuated equilibria – suggested by Stephen Gould, evolution occurs via long periods of stability or stasis punctuated by periods of rapid change.

The Timing of Evolution

Gradualism Punctuated Equilibrium

The Timing of Evolution and

Genetics

Change in the timing of growth and development -- heterochrony, can be responsible for changes in the shape or size of a body part.

There are also homeobox genes that are responsible for large-scale effects on the growth and development of the organism. They may be linked to patterns of evolutionary timing predicted by the punctuated equilibria model.

Visual Counterpoint: For

Class Discussion

The differences between chimpanzees and humans are sometimes linked to a type of heterochrony known as neotony (the retention of juvenile traits). Which juvenile chimp traits do you think we have retained?

Constructing Evolutionary Relationships

Evolutionary relationships can be traced through:

(1) homologous traits = originate from a common ancestor

(2) analogous traits = reflect similar environmental pressures not ancestry

Constructing Evolutionary Relationships

Constructing Evolutionary Relationships

The legs of prosimians and humans do not have a close evolutionary relationship; they are a result of the convergent evolution of homologous traits.

Constructing Evolutionary Relationships

In addition to homologous and analogous traits, evolutionary relationships can be traced through:

(1) Derived traits = traits found in a group of organisms that did not exist in ancestral populations.

(2) Ancestral traits = traits possessed by an organism or group of organisms due to shared ancestry.

Macroevolution of the Early Primates

Continental Drift and Geological Time

Because primate evolution extends so far back in time, paleoanthropologists reconstruct primate evolution in conjunction with information about the geological history of the earth.

Continental Drift and Geological Time

Continental drift = According to the theory of plate tectonics, the movement of continents embedded in underlying plates on the earth’s surface in relation to one another over the history of life on earth. Such changes can also affect the climate and environment.

Early Mammals

Mammals appear as early as 190 million years ago.

Around 70 mya, the earth began to experience sever climatic changes and many of the larger reptiles and dinosaurs went extinct, opening new environmental niches for mammals.

Mammal-like animals were preadapted to advantages of these changes and underwent an adaptive radiation -- a rapid increase in the number of related species following a change in their environment.

Early Mammals: Adaptive Traits

• Mammals are homeotherms - animals that maintain a relatively constant body temperature despite environmental fluctuations.

• By contrast, reptiles are isotherms -animals whose body temperatures rise or fall according to the temperature of the surrounding environment.

Early Mammals: Adaptive Traits

• Mammals are k-selected – reproduction involves the production of relatively few offspring with high parental investment in each.

• Other animals are more r-selected - reproduction involves the production of large numbers of offspring with relatively low parental investment in each.

The Proto-Primates (65 mya)

The first primate-like mammals, or proto-primates, were roughly similar to squirrels and tree shrews in size and appearance. 

The existing, very fragmentary fossil evidence (mostly from North Africa) suggests that they were adapted to an arboreal way of life in warm, moist climates. 

The Rise of the Primates

The rise of primates appears to be an adaptive radiation linked to the spread of flowering plants (angiosperm radiation) and pollinating insects.

The Rise of the Primates

(1) arboreal hypothesis – a hypothesis for primate evolution that proposes that life in the trees was responsible for enhanced visual acuity and manual dexterity in primates.

(2) visual predation - a hypothesis for primate evolution that proposes that hunting behavior in tree-dwelling primates was responsible for their enhanced visual acuity and manual dexterity.

Timeline of Primate Evolution

True Primates - Early Prosimians

Their bones have been found in 55 million year old geological deposits in North America-Europe-Asia (Laurasia) and Africa.

The rainforest environments were warm and wet.

They were somewhat squirrel-like in size and appearance, but apparently they had grasping hands and feet that were increasingly more efficient in manipulating objects and climbing trees.

True Primates - Early Prosimians

Early prosimians had postorbital bars and relatively long snouts.

Early Strepsirhines

Fossils of lemur and loris ancestors are found in North America, Europe, Asia, and possibly Africa.

Main Features: small eye sockets, elongated snouts, cheek teeth adapted for folivorous or frugivorous diets, and relatively large body mass (greater than 1 kg).

Early Haplorhines Fossils of haplorhines

(ancestors of tarsiers and anthropoids) are found in North America, Europe, Asia, and possibly Africa.

Main Features: large eye sockets, shortened snouts, cheek teeth adapted for insectivorous or frugivorous diets, and relatively small body mass (less than 500 g)

Oligocene Anthropoids

By the Oligocene period, prosimians are out-competed by anthropoids (except in Madagascar).

Deposits in Fayum (Egypt) and other parts of North Africa, show the development of anthropoids some with the dental formula similar to Platyrrhines (2-1-3-3) and others with the Catarrhine dental formula (2-1-2-3)

Aegyptopithecus

the size of a large domestic cat (13-20 pounds)

fruit and seed eating forest tree-dweller

had fewer teeth, less long snout, larger brains, and increasingly more forward-looking eyes than the prosimians

• The earliest platyrrhine fossils are found in South America and are only about 25 million years old.

• Both platyrrhines and catarhines originated from prosimian ancestors – although anthropologists still debate if it was the same ancestor.

• A possible African origin for platyrrhines is suggested by the “rafting” and “island hopping” hypotheses but the geological evidence for these is inconclusive.

The Origin of New World Monkeys

Miocene Apes

Evolutionary relationships are based on a molecular clock - a hypothesis that dates of divergences among related species can be calculated through an examination of the genetic mutations that have accrued since the divergence.

Proconsul

• existed from 27 to 17 million years ago in Africa

• ranged in size from 22-84 lbs (10-40 kg)

• its monkey-like features included: thin tooth enamel, a light build with a narrow chest and short forelimbs, and an arboreal quadrupedal lifestyle

• its ape-like features are its lack of a tail, ape-like elbows, and a slightly larger brain relative to body size

Proconsul

Dryopithecus

• existed from 12 to 9 million years ago in Eastern Africa and Eurasia

• had a Y-5 cusp arrangement on its molar teeth – typical of hominoids

Dryopithecus

Sivapithecus

• existed from 12.5 to 8.5 million years ago in India and Pakistan

• possible ancestor of the orangutan and gigantopithecus – already had separated from the common ancestor of chimpanzees and gorillas

Sivapithecus

Gigantopithecus

• existed from 1.0 million to 300,000 years ago in China, India and Vietnam

• it stood about 10 feet (3 meters) tall and weighed as much as 1200 pounds (545 kg)

• Gigantopithecus was probably a quadruped and an herbivore, existing on a diet primarily of bamboo, possibly supplemented with seasonal fruits

Gigantopithecus