In 1831, an inexperienced 22-year-old British naturalist named Charles Darwin jumped on the HMS Beagle and sailed the world on a five-year scientific voyage that earned him a place in science and history.

Known today as the “father of evolution,” Darwin amassed compelling evidence supporting the theory of evolution by natural selection. Earlier scholars, including his grandfather Erasmus Darwin, were mocked for presenting such unorthodox ideas as transmutation of species.

Darwin is credited with being the first scientist to persuasively argue a unifying theory of how species evolve and continue to change.

Charles Darwin grew up on an idyllic English estate where he spent his days collecting rare beetles, moths and fossils. His love of nature persisted despite his father’s insistence that young Charles pursue a practical career in medicine at the renowned University of Edinburgh. Not to be deterred, Charles found a mentor in marine biologist Robert Grant and immersed himself in natural science.

Grant introduced Darwin to the idea that life sprung from a common ancestor by pointing out similarities between a human hand and a bird wing. Two years later, Darwin transferred to another school where he focused on botany.

His first professional job was working as a naturalist on the HMS Beagle, a survey boat that took him to exciting places like Brazil, Argentina, the Canary Islands, the Galapagos Islands and Sydney, Australia.

Darwin was influenced by the work of geologist Charles Lyell, who believed in the principle of uniformitarianism. Darwin and Lyell considered fossil records and striated layers in rock formations as evidence of slow and continual change. Darwin applied his knowledge of variation in plants, animals, fossils and rocks to the origin of the species by means of natural selection.

Religious beliefs and science were closely intertwined in Victorian England. The Bible was the respected authority on how and when life on Earth was created by God. Many scientists acknowledged that species change over time but couldn’t comprehend how or why living organisms change once they appear.

French naturalist, Jean Baptiste Lamarck, was a pioneer in evolutionary theory who challenged the notion that species were immutable based on fossil records. He argued that traits could be acquired and passed along to the next generation.

For instance, Lamarck thought that so-called “nervous fluid” was secreted when giraffes reached for leaves, producing a longer neck that would be inherited by the next generation. Lamarck was ostracized for his suggestion that natural processes, rather than a divine design, determined the direction of life.

The 19th century was a turning point in how people viewed the history of life. Great minds from multiple disciplines influenced one another’s theories. Darwin followed the work of progressive thinkers of his time, such Thomas Malthus. A political economist, Malthus argued that people and animals overproduce and put a drain on resources. He advocated for regulation of family size as a means of population control.

Darwin saw some logic in Malthus’ arguments and applied the concept of overpopulation to the natural world. Darwin reasoned that animals compete for survival from the moment of birth.

When resources are scarce, competition is intense. Random, naturally occurring variations make some siblings more fit than others to successfully compete, mature and multiply.

In the 1850s, Alfred Russel Wallace collected thousands of exotic specimens and noticed regional differences in traits. He concluded that the best-suited organisms for a region were naturally more likely to survive and pass along their characteristics. Wallace shared his ideas with Darwin, who had been collecting evidence of natural selection for a much longer time.

Darwin had held off releasing his findings for fear of public ridicule. However, he did not want to see Wallace receive all the credit if the idea of national selection was favorably received. Soon after, Darwin and Wallace simultaneously presented their work to the Linnaean Society.

A year later, Darwin published his groundbreaking work On the Origin of Species.

Darwin defined evolution as a process of “descent with modification.” He believed that some organisms within a species have trait variants that make them fitter and more likely to reproduce.

Over time, inherited modified traits become dominant in the population, and a new species may emerge. Taking the idea further, Darwin speculated that all life evolved from one common ancestor millions of years ago.

Descent from modification also explains extinction. Certain characteristics may be crucial to plant survival, such as thorns. In a heavily grazed area, plants without thorns could be consumed before they go to seed.

Traits acquired during the lifetimes of those eaten plants are not passed along to any offspring, with the exception of gene mutations in sex cells, such as exposure of germ cells to damaging radiation.

Darwin’s theory of evolution by natural selection solved the mystery of how evolution works. Darwin figured out that that certain traits and characteristics are better suited to the environment, which enables organisms with the adapted variant to better survive and multiply.

Slowly, over time, a once uncommon gene variant may eventually become the predominant gene in the population via natural selection.

Survival of the fittest is another premise of Darwinian evolutionary theory. However, this does not mean the biggest, fastest and toughest always win. Fitness is a fluid concept relative to the traits needed for survival at a particular time and place. Biodiversity makes a population stronger because change is ongoing, and the evolutionary process keeps pace.

Fossil records provide compelling evidence of the evolutionary history of living things. Gradual, incremental changes in land and marine fossils coincide with climate change or migration.

For example, the modern-day horse once looked more like a fox. Paleontologist can show how the ancient horse adapted by slowly acquiring hooves, height and flat teeth as an adaptive modification to living on open grasslands instead of the forest.

DNA extracted from the recovered bones and teeth of Neanderthals indicates that modern humans and Neanderthals descended from the same ancestral group, as supported by DNA sequence analysis. The Neanderthals moved out of Africa and hunted mammoths during the Ice Age.

Later, Homo sapiens and Neanderthals crossed paths again and had children together. Neanderthals died out, but many people today have Neanderthal gene variants in their human genome.

The now extinct Tiktaalik is an example of a missing link that shows when species evolved in very different directions. Tiktaalik was a large fish with characteristics of an amphibian, including a flat head and a neck. Around 375 million years ago, this “fishapod” adapted to living in shallow water and land. Tetrapods, or animals with four feet, descended from these primitive amphibians.

Vestigial organs, like the human appendix, are remnants of a body part that once served a purpose. For instance, vestigial tails in humans are an unusual evolutionary throwback that occurs when the tail of the embryo fails to dissolve properly. Normally, the tail of the human embryo forms the coccyx (tailbone). On rare occasion, a baby will be born with a tail that may be fleshy or bony, and a few inches long.

According to the American Museum of Natural History, the small hind leg bones under the skin of boa constrictors and pythons reflect the snakes' evolutionary history. Boa constrictors and pythons descended from lizards that happened to be born with stubby legs. Short legs were better for survival than long legs in certain environments.

Genes for short legs became dominant in the population, and eventually legs disappeared except for unseen vestigial bones near the snakes' tails.

While traveling the world on the HMS Beagle, Darwin was enthralled by the many different types of island finches. He noted that the finches had various adaptations to suit their environment, like changes in beak size and shape depending on the food they ate.

Darwin’s finches are a textbook example of adaptation and evolution on a small scale. Birds had migrated to the islands from the mainland, and species gradually evolved to fit their new environments. Natural selection happens because organisms in a population typically have randomly occurring gene variations and mutations that affect adaptation.

Evolution requires existing variation in the species. For example, giraffes with a random variation of an unusually long neck were better able to reach leaves in the canopy, making them fitter to survive and more likely to reproduce. Offspring with the same variation of a longer neck enjoyed the same evolutionary advantage at feeding time. The giraffe evolved over time to have the characteristic long neck seen today.

Darwin’s ideas offended Christians who believed that God created the universe and made man in his image and likeness. The very suggestion that humans, worms and whales had a common ancestor seemed laughable at a time when DNA was not known or understood.

Although some questions remain, the theory of evolutionary change is widely accepted by scientists around the world now. The creationist view of human evolution is generally considered to represent a religious belief based on faith rather than a scientific theory.

Darwin’s findings resulted from years of painstaking work classifying living organisms based on observed traits, behavior, vocalizations and overall appearance. He was able to develop his theory of evolution without knowing the exact mechanism behind it. The discovery of genes and alleles answered the question that Darwin could not solve.

Descent with modification is the result of gene recombination and mutations in germ cells that get handed down to the next generation. Genetic changes resulting from mutations can be harmless, helpful or detrimental. Genetic variations and modifications in populations often lead to the emergence of new species.

A common ancestor is based on remarkable similarity in genetic material, genetic codes and gene expression. Cells of multicellular organisms grow, metabolize, divide and mutate much the same way. Molecular biology allows comparison of organisms and species at the cellular level.

Closely related organisms have similar sequences of amino acids in their genes. Certain genes may be almost identical in different species as the result of sharing a common ancestor. Humans and chimpanzees have an almost identical gene that encodes insulin.

Humans and chickens both code for insulin, but the genes have fewer similarities, revealing that humans are more closely related to monkeys than fowl.

Humans continue to evolve as a species. Blue eyes came about just 10,000 years ago when a gene mutation turned off the switch to produce brown eyes. Other relatively recent mutations include an ability to digest milk. The process of natural selection and survival of the fittest may have a more limited effect on modern human evolution, however.

Advances in modern medicine make it possible to survive diseases that would have once proved fatal. Many people are having babies when they are older, when the risks of genetic diseases may be greater. The theory of evolution holds that life will continue to diversify and adapt to changing conditions.