Evolution: Natural Selection

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. 

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.

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.

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. 

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.

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.

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.

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.

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.

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.

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.

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.

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.

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. 

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.

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.

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.

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. 

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

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.

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.

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.

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. 

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.

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.

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.

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.