Evolution: Natural Selection

Evolution is the process of genetic change over a long period of time. Natural selection is the mechanism by which evolution takes place.

Senckenberg Naturmuseum Frankfurt, The Field Museum Chicago, Royal Museum of Natural Sciences Belgium, American National History Museum, and more.

In this Expedition, you will tour natural history museums and marine reserves around the world in order to understand how natural selection works and to discover how individual species of plants and animals have evolved over millions of years.


Most species in the order Proboscidea are extinct. The species that survive—the elephants—are the largest land animals on Earth. The order gets its name from proboscis, the Latin word for nose

Proboscidean’s unique noses are made of muscles so strong that they can lift logs and so sensitive they can pick up peanuts. Proboscidea also have pairs of sharp tusks. Their strong legs are as thick as tree trunks. Their large ears serve many functions besides hearing, such as signaling other elephants and cooling the body.


Mastodons are an extinct species of Proboscidea. They roamed North America up until about 11,000 years ago. Scientists know that Native Americans hunted mastodons because spear points have been found inside mastodon skeletons. Mastodons also inhabited parts of Asia and Europe.


The African bush elephant (left) and Asian elephant (middle) are living elephant species. The extinct dwarf elephants (right) inhabited islands in the Mediterranean Sea. With their limited food capacities, the island habitats supported dwarf elephants but not larger species.

Survival and Extinction

Over millions of years, the Proboscidea evolved into many species. Some species had characteristics that enabled them to survive when their environments changed. Other species lacked such adaptations; these species became extinct.

Evolution at The Field Museum

The Evolving Planet exhibit at the Field Museum, Chicago, includes a timeline that shows how and when life on Earth began. Over billions of years, life forms evolved from single-celled organisms into the multicellular ancestors of today’s plants and animals. 

The exhibit describes the processes that allowed life to thrive and evolve: photosynthesis, reproduction, genetic variation, and natural selection.


Photosynthesis began evolving about four billion years ago, when single-celled cyanobacteria (blue-green algae) began producing oxygen. In photosynthesis, green plants and algae use energy from sunlight to take in carbon dioxide, build carbon-based cells, and release oxygen into the atmosphere.

Genetic Variation and Natural Selection

Living things in any species have individual genetic characteristics. Individuals with variations that favor survival in a given environment have the best chance of passing on their genes. 

Over succeeding generations, this process of natural selection changes the genetic characteristics of the species as a whole.

First Animals

About 600 million years ago, the genetic ancestors of today’s animals began to evolve. These multicellular creatures had specialized cells to perform specific functions, such as feeding, excretion, and reproduction.

“An Explosion of Life”

During the Cambrian and Ordovician periods (543 to 443 million years ago) Earth was covered by warm, shallow seas. This environment was highly favorable to living things. Green plants and many forms of animals evolved at a relatively rapid rate.

Human Evolution at the Royal Museum

The Gallery of Humankind at Belgium’s Royal Museum of Natural Sciences presents the evolution of humans over seven million years and more. The exhibit includes 25 different hominids, including our own species, Homo sapiens. 

We are the result of a long physical, cultural and social evolution, having undergone major adaptations such as bipedalism, increasing dexterity, brain growth, and shrinking teeth. 

In this gallery, you’ll explore some species that were our direct genetic ancestors and others that have become extinct.

Homo heidelbergensis

This hominid species lived in Europe, Africa, and possibly China, between 700,000 and 200,000 years ago. It was the first species to hunt large animals on a regular basis and one of the first to use fire. It was also the first species to build shelters.

Just Us Folks (Homo sapiens)

Our own species of hominids evolved in Africa about 200,000 years ago. We have larger brains and smaller teeth than previous hominid species and are far more socially advanced. We are the only surviving hominids.

Neanderthals (Homo neanderthalensis)

This hominid species lived in Europe and Asia between 400,000 and 40,000 years ago. Neanderthals made relatively advanced tools, used fire, and built shelters. They also made clothing and decorative objects and buried their dead.

Homo georgicus

This hominid species lived in Eastern Europe as early as 1.8 million years ago. It is the oldest known species of hominids in Europe. Archeologists have found the remains of many individuals and expect to discover more about human society by studying these fossils.

Homo erectus

This hominid species lived in Africa and Asia between 1.89 million and 143,000 years ago. It was the first species that walked upright on two legs and one of the first to spread beyond Africa. Fossils show that these hominids used stone tools and cared for weak and elderly individuals.

The Hobbit (Homo floresiensis)

This hominid species lived in Indonesia about 100,000 to 50,000 years ago. The species used stone tools to hunt animals such as large rodents. Their nickname comes from their physical characteristics: they were small in stature, with relatively big teeth and feet.

Comparing Chimps and Humans

The Spitzer Hall of Human Origins at the American Natural History Museum in New York City displays fossils, models, and life­sized tableaux that explain human evolution. 

DNA, or deoxyribonucleic acid, is a chemical compound that contains the genetic code of living things. Two primates, the chimpanzee and the bonobo, a species of ape, are humans’ closest genetic relatives. 

Parts of the hall compare the similarities and differences between humans and other primates such as chimpanzees, including comparisons of DNA. 


Although humans and chimps share 98.8 percent of their DNA, the species differ a great deal. In fact, because genes can form so many combinations, that 1.2 percent of divergence results in about 35 million differences.

The Big Catastrophe

About 66 million years ago, at the end of the Cretaceous (K) Period, in a huge mass extinction, about 80 percent of Earth’s species, including the large dinosaurs, died off. Scientists still do not agree on what caused the K-T extinction. 

One theory is that an asteroid or comet hit the Earth. Increased volcanic activity is another possible cause. Many species survived the ecological disaster. Flowering plants, insects, birds, corals, and fish all diversified greatly during the next geological era, the Tertiary (T) Period.

Species Survival and Extinction

At the time of the K-T extinction, Earth’s environments changed dramatically. Life forms with adaptations for the new environments survived, reproduced, and passed on their genetic traits; species without such adaptations became extinct. This is an example of natural selection.

The Tree of Life

This interactive tree of life, displayed in the Australian Museum, shows the genetic connections between Earth’s life forms, from simple one-celled organisms to complex plants and animals, including human beings.

The exhibit’s interactive section focuses on animal life, using specific animals as examples. In general, the tree progresses from earlier, simpler life forms on the right to more recent and more complex organisms on the left.

Genetic Similarities and Differences

The genetic codes of life forms are passed along in a branching, not linear manner. So although humans share some genes with sponges and worms, they share many more genes with other mammals, which are closer to humans on the tree of life.

Evolution of the Horse

System Earth, an exhibit at Berlin’s Museum of Natural History, shows how Earth’s systems and organisms interact with each other. The large interactive globe demonstrates how plate tectonics, volcanism, meteorite impacts, and mountain building affect Earth’s atmosphere and environments. 

The surrounding exhibits explain the effects of such events on the evolution of life forms. Horses evolved from small creatures that inhabited rainforests to larger animals that inhabited grasslands. Horses encouraged the spread of grasslands by preventing invasions by species of bushes.

Przewalski's horse

Przewalski's horses are the last surviving subspecies of wild horse. They are also among the genetic ancestors of today’s domestic horses. 

The modern horse, Equus caballus, evolved in central Asia and spread through Europe and North America. The North American species became extinct between 10,000 and 8,000 years ago.

The Dawn Horse

The dawn horse, also known as Eohippus, belonged to the genus Hyracotherium. This now-extinct genus lived during the Eocene epoch, between 55.8 and 33.9 million years ago. Eohippus was a very small animal, only one or two feet (33–66 cm) tall.

Earth’s Story from the Trees

The Hall of North American Forests at the American Museum of Natural History in New York City explores different types of forests in various locations, including Colorado, South Carolina, and the Great Smoky Mountains. 

The displays show how each type of forest evolved and describes the flora and fauna that flourish in forest habitats. A diorama of a forest floor shows the bugs, beetles, and weevils that take part in the processes of decomposition and soil regeneration.

Giant Sequoia

The giant sequoia, or Sierra redwood, is the largest and longest-living tree on Earth. The concentric rings on this giant slice from an ancient sequoia tree record more than 1,400 years of history. The rings show variations in climate, which resulted in changes in the rate and types of growth.

Some Steps

The David H. Koch Hall of Human Origins at the Smithsonian National Museum of Natural History uses a variety of exhibits to explain human evolution over six million years plus. 

Scientists do not agree on how many early human species there were, how the species were related to each other, or exactly when early humans developed specific traits.

In general, the traits that we consider uniquely human—large brains, complex social behavior, art and language—developed during the last 100,000 years.


Lucy is the nickname for a skeleton discovered in Ethiopia in 1974. The skeleton is about 3.2 million years old. Lucy had relatively long arms and a broad pelvis that allowed her to walk upright. Her long, curved toes show that she was also able to climb trees.

First Step: Walking Upright

Our genetic ancestors evolved bipedalism (the ability to walk upright on two legs) over 4 million years ago. Evidence for bipedalism includes trace fossils of ancient footprints, which allow scientists to estimate the height and weight of the early humans who made them.

Adaptation to Climate

The Smithsonian’s Human Origins Program is studying the evolutionary responses of humans to climate change. This research looks at the adaptations that allowed some species to survive in changing climates, as well as the adaptations in the flora and fauna on which humans depended.

Hunting (Creating Tools)

Over 2.6 million years ago humans were already using stone tools. By 500,000 years ago, they were making long spears that could kill large animals. Spears and other complex weapons increased the amount of meat in the human diet.

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