The 19th century was a time of groundbreaking scientific discoveries that upended many previously held theories about the origin of Earth and mankind. In 1855, Alfred Russell Wallace published his proposal of a theory of evolution by way of natural selection, followed by Charles Darwin’s 1859 published work On the Origin of the Species.

Years of work gathered compelling evidence that led to wide acceptance of the theory of evolution by scholars around the world.

Naturalist Charles Darwin spent years analyzing the evidence of evolution before publishing his findings. His theory was heavily influenced by like-minded scholars of the time, particularly Alfred Russell Wallace, James Hutton, Thomas Malthus and Charles Lyell.

According to the theory of evolution, organisms change and adapt to their environment as a result of inherited physical and behavioral characteristics passed down from parent to offspring.

Darwin’s definition of evolution centered on the idea of slow and gradual change over repeated generations, which he called “descent with modification.” He proposed that the mechanism of evolution was natural selection. Darwin’s observations led him to conclude that trait variations within a population afford certain living organisms a competitive advantage for survival and reproduction.

The evidence of evolution’s definition draws heavily from the biogeographical studies of Wallace in the Amazon rainforest and Darwin’s observations on the pristine Galapagos Islands. Both researchers defined evolutionary evidence as proof of a link between living organisms and their common ancestor.

Exciting discoveries in the Galapagos Islands provided Darwin with a solid foundation for pressing the idea of evolution and natural selection. For instance, Darwin noted different beak variations within the natural population of Galapagos finches, and later came to understand the importance of his findings. Darwin discerned that the different species of finches descended from a South American species that had migrated to the Galapagos.

Darwin’s conclusions were corroborated in recent studies conducted by climatologists Peter and Rosemary Grant. The Grants traveled to the Galapagos Islands and documented how changes in temperature altered the food supply. Consequently, certain types of species died off while others survived, thanks to particular trait variations in the population, such as long, probing bills to reach insects.

Natural selection leads to survival of the fittest, meaning that better-adapted organisms edge out less-adapted species. Examples of selection pressures include:

• Amount of available food
• Shelter
• Climate change 
• Number of predators 

Inherited modifications accumulate and can result in the emergence of a new species. Darwin argued that all living things descended from a common ancestor over millions of years.

1. Fossil Evidence

Paleoanthropologists have traced the history of human evolution by analyzing fossilized bones that show how brain size and physical appearance slowly changed. According to the Smithsonian National Museum of Natural History, Homo sapiens (modern humans) are primates closely related to the great apes of Africa and share a common ancestor that existed around 6 to 8 million years ago.

Fossil records can date organisms from certain time periods and show the evolution of different species from a common ancestor. Fossil records are often compared to known facts about the geology of the area where the fossils were located.

2. Discovery of Ancestral Species

Darwin's fossil-hunting treks provided considerable evidence for evolution and the existence of extinct ancestral species.While exploring South America, Darwin found remains of an extinct type of horse.

The ancestors of modern American horses were small grazing animals with toes on their feet that shared a common ancestor with a rhinoceros. Adaptations over millions of years included flat teeth for chewing grass, increased size and hooves for running swiftly from predators.

Transitional fossils can reveal missing links in the evolutionary chain. For instance, the discovery of the genus Tiktaalik potentially shows fish evolution into land animals with four limbs. In addition to being a transitional species with gills, the ancestral Tikaalik is also an example of mosaic evolution, meaning its body parts evolved at different rates when adapting from water to land.

3. Increasing Complexity of Plants

Grass, trees and mighty oaks evolved from a type of green algae and bryophytes that adapted to land about 410 million years ago. Fossil spores suggest that primitive algae adapted to the dry air by developing a protective cuticle coating for the plant and spores.

Eventually, terrestrial plants developed a vascular system and flavonoid pigments for UV protection from the sun. The reproductive life cycle in multicellular plants and fungi became more complex.

4. Similar Anatomical Features

The theory of evolution is bolstered by the existence of homologous structures, which are shared physical traits between multiple species, showing they descended from a common ancestor.

Almost all limbed animals have the same structure, which suggests shared traits before diversifying from a common ancestor. Similarly, insects all start out with an abdomen, six legs and antennae, but diversify from there into a huge number of species.

5. Gills in Human Embryos

Embryology offers powerful evidence supporting the theory of evolution. The embryonic structure that living organisms share is virtually identical between species going back to a common ancestor.

For instance, embryos of vertebrates, including human beings, have gill-like structures in the neck that are homologous with fish gills. Certain ancestral characteristics like gills on an embryonic chicken do not develop into an actual organ or appendage, however.

Embryology offers powerful evidence supporting the theory of evolution. The embryonic structure that living organisms share is virtually identical between species going back to a common ancestor.

For instance, embryos of vertebrates, including human beings, have gill-like structures in the neck that are homologous with fish gills. Certain ancestral characteristics like gills on an embryonic chicken do not develop into an actual organ or appendage, however.

6. Odd Vestigial Structures

Vestigial structures are evolutionary leftovers that served a purpose for a common ancestor. For example, human embryos have a tail in the early stages of development. The tail becomes an indistinguishable tail bone because having a tail would serve no useful purpose in humans. Tails in other animals help them with different functions like balance and swatting flies.

The vestiges of hind leg bones in boa constrictors are evidence of evolution of lizards to snakes. In some habitats, lizards with the shortest legs would have been more motile and harder to see. Over millions of years, the legs became even shorter, and almost non-existent. The common phrase, “Use it or lose it” also applies to evolutionary change.

7. Research in Biogeography

Biogeography is a branch of biology that supports Darwin's theory of evolution. Biogeography looks at how the geographic distribution of organisms around the world adapt to different environments.

Geography plays a pivotal role in speciation. Darwin’s finches diversified from finch ancestors on the mainland and between the Galapagos Islands to fit their current surroundings. Ancestral species of finches were seed eaters that nested on the ground; however, the finches discovered by Darwin nested in various places and fed on cactus, seeds and insects. Beak size and shape directly related to function.

Kangaroo Island near Australia is one of the few places on Earth where marsupials flourish along with placental mammals and egg-laying monotremes. As the name suggests, marsupials like kangaroos and koalas thrive and vastly outnumber the human inhabitants.

After the island separated from the Australian continent, the flora and fauna evolved into subspecies undisturbed by animal predators or colonization until the 1800's. Scientists compare and contrast plants, animals and fungi of the mainland with those found on Kangaroo Island to learn more about adaptation, natural selection and evolutionary change.

Random variations in plants and fungi made some organisms better suited to colonize a new area and pass along their genetic code, thereby, supporting Darwin's theory of natural selection.

8. Analogous Adaptation

Analogous adaptation lends support to the process of natural selection and the theory of evolution. Analogous adaptations are survival mechanisms adapted by unrelated organisms facing similar selection pressures.

The unrelated Arctic fox and the ptarmigan (polar bird) go through seasonal color changes. The Arctic fox and the ptarmigan have a gene variation that allows them to develop a lighter color in the winter to blend in with the snow and evade hungry predators, but that does not indicate a common ancestor.

9. Adaptive Radiation

Hawaii is a chain of islands where many spectacular birds and animals can be found that are believed to have originated in East Asia or North America.

About 56 different species of Hawaiian honeycreepers evolved from just one or two species, which then settled in different microclimates on the island in a process called adaptive radiation. Variations in Hawaiian honeycreepers show many of the same type of beak adaptations as Darwin’s finches.

10. Post-Pangaea Species Divergence

Millions of years ago, the Earth’s continents were close together and formed a supercontinent called Pangaea. Similar organisms could be found all over the world. The shifting plates of the Earth’s crust caused Pangaea to drift apart.

Flora and fauna evolved differently. The plants, animals and fungi from the original landmass evolved differently on the newly formed continents. Ancestral lineages evolved into new lineages post-Pangaea as organisms adapted to geographic changes.

11. DNA Proof

All living organisms are made up of cells that grow, metabolize and reproduce according to their genetic code. The unique blueprint of an entire organism is contained in the cell’s nuclear deoxyribonucleic acid (DNA). Examining the DNA sequences of amino acids and gene variants of animals, plants and fungi gives clues to ancestral lineage and a common ancestor.

DNA kits can reveal ancestry and identify long-lost relatives based on comparison of genetic material in submitted samples of saliva or cheek swabs. Genetic variance in a natural population is the result of normal gene shuffling in sexual reproduction and random mutations during cell division. Uncorrected mistakes can result in problems such as too many or too few chromosomes, resulting in genetic disorders.

More often, mutations are inconsequential and do not affect gene regulation or protein synthesis. Occasionally, a mutation may turn out to be an advantageous adaptation.

The evolutionary history of living organisms, including human origins, dates back millions of years. However, you can find evidence of fast and quick evolution of different species. For example, bacteria rapidly reproduce and evolve to have antibiotic-resistance genes.

Insects that are better able to resist pesticides survive and reproduce at a higher rate.

Examples of natural selection are recognizable in real time. For instance, light-colored field mice are easily spotted in a cornfield and eaten by predators. Brownish gray mice are better able to blend into their surroundings. Camouflaged coloring enhances survival and reproduction.

Evolutionary theory has useful applications in agriculture. Even before genes and DNA molecules were discovered, farmers used selective breeding to improve crops or a livestock herd. Through the process of artificial selection, plants, animals and fungi with superior qualities were and are crossed to improve the overall population and create ideal hybrids.

However, hybrids often have little variability, which threatens the species' survival if environmental conditions change or disease strikes.