It is well understood that our species, Homo sapiens, evolved in Africa. Many hominid species shared a common ancestor with the apes. They all evolved in the region of the Rift Valley in Africa some 7 million years ago. Of these, humans are the only ones still on Earth today. There was never a first human, only slowly evolving populations of hominids.

We are the only living hominid species on Earth. Our parents, grandparents, and great-great-great grandparents were humans, just like us. How, then, are we related to our ancestor species? If we were to draw a line and trace our lineage back 100 generations, the individual we find there, let's call them #100, is a human who lived in Egyptian times. Individual #1,000 in this line is also human, like those who painted the walls of the Chauvet Cave in France or the Cave of Altamira in Spain.

If we go back 10,000 generations to individual #10,000, we will still find a human being, but we begin to see a difference. #10,000 is a Paleolithic human from the Old Stone Age. They are a contemporary of Omo I, the oldest known fossil of Homo sapiens.

Individual #75,000, from 75,000 generations ago, is no longer human. He is a member of the species Homo erectus, a close relative of the famous fossil, Java Man. He used hand axes known as Acheulean axes to butcher the animals he hunted. Their stone tools are named after the place where the ax was first discovered in the 19th century: Saint Acheul, Amiens, France.

The female #120,000 is an australopithecine, a relative of Lucy, Australopithecus afarensis. She was short in stature and had very long arms used to swing from trees. Three million years separate us. She lived in Africa. Most of her contemporaries disappeared without leaving descendants, but she was among the fortunate few who managed to pass on her genes to future generations.

Individual #1,500,000 lived 25 million years ago. This human ancestor resembled the first apes, like Proconsul. They lived in the trees, seeking protection from the lions, leopards, and cheetahs that hunted them.

We can continue meeting our ancestors as we go further back in time. At 375 million years ago, we meet a freshwater fish known as Tiktaalik. We saw our first vertebrate ancestor during the Cambrian Era 525 million years ago. Our first multicellular ancestor lived 600 million years ago. Back in time we go, until we find the last universal common ancestor of all life on Earth, LUCA, 3.5 to 3.8 billion years ago.

All along this line of ancestors, we will never find an individual who is a different species from their parents or their offspring. However, all individuals will have a distant ancestor who is very different from them. Everyone in the line is very similar to their neighbors on either side, but as we move further and further away, more and more differences will appear. Imagine a person taking a photograph every day of their life. The earliest photos will be very different from the most recent ones. However, it is impossible to stop and choose the one photograph that represents the moment the child becomes an adolescent or the adolescent becomes an adult.

The evolution of hominids from their ape-like ancestors six million years ago to the present is well understood, but there are currently no ape or hominid fossils dating from 7 to 10 million years ago. The climate was humid and warm, like a tropical rainforest, which accelerated the decomposition of organic material long before it could fossilize. However, scientists estimate that the australopithecine lineage, which includes Ardipithecus and Australopithecus, separated from the ape lineage at the end of the Miocene Period between 6 and 7 million years ago.

The Miocene Epoch was characterized by major global climatic changes with increasingly colder winters north of the Equator. By the late Miocene, evergreen broad-leaved forests were replaced by open woodlands, shrublands, and grasslands in many regions inhabited by ape-like primates. These environments stimulated novel adaptations involving locomotion. Some of the quadrupedal apes trapped in forested areas adapted to walking on two legs on land but continued to use all four limbs to move through the trees. That is, they demonstrated both terrestrial bipedalism and arboreal quadrupedalism. These adaptations gave rise to the australopithecines. Then, 2.5 million years ago, another global cooling event led to the creation of the vast African savannahs we are familiar with today. The hominins who were already partially bipedal adapted to a fully bipedal life.

If these climatic changes had not happened, the descendants of the apes would not have needed to move about on two legs, and the genus Homo might not have appeared.

Thus, we can confidently state that we result from successive evolutionary steps in a long series of ancestor species. Our human traits evolved slowly over a very long time. Here are some examples:

• Rhodopsin is the pigment-containing sensory protein that converts light into an electrical signal in our retinas. It is present in all visual systems, from the simplest, such as bacteria and archaea, to the most complicated. It is more than two billion years old.

• Our arm bones, the humerus, ulna, and radius, are 380 million years old. They appeared in the fish, Eusthenopteron, and predate the appearance of tetrapods.

• Our hand structure is 375 million years old and can be seen in the fins of Tiktaalik.

• Two of the three bones in our middle ear, the hammer and anvil, are 200 million years old. They evolved from the jaw bones of reptiles. The third tiny bone in our middle ear, the stirrup, can also be found in the middle ear of reptiles.

• Our primate nails are 55 million years old. They are the equivalent of flattened claws.

• We lost our tails 23 million years ago when it was no longer beneficial to the common ancestors of all modern apes. Animals have used tails for millions of years for various functions, including locomotion in fish, balance in felines, or grasping in New World monkeys.

• Our thumb is around 5 million years old and is the only fully opposable thumb in the Primate order. Longer than any other, the hominin thumb played a critical role in our evolution. It helped our ancestors grasp and manipulate objects, turning them into useful tools. This simple act sets us apart from many other hominids.

• Our pelvis and knees are very different from those of our closest living relatives, the great apes. The differences trace back to when our ancestors became bipedal three to 4 million years ago. The discovery of the fossil Ardipithecus Ramidus, Ardi, in 1994 confirmed this hypothesis.

• Our feet, which lack an opposable big toe, were present in Homo habilis 2.5 million years ago.

• Our large forehead and brain (on average 1400 cc) are between 1 and 2 million years old.

• One of the most recent human features is the chin, around 200,000 years old. We are the only hominid with this bony protuberance below the labial area. Scientists hypothesize that chins evolved as our species became less aggressive and more sociable, possibly due to changes in our species' hormone levels.

The combination of traits that identify us as human beings today results from the many adaptations found in our long line of ancestor species. Human beings did not appear overnight; we evolved step-by-step over hundreds of millions of years.

Darwin used fossils to defend his theory, but he also knew that the fossil record was imperfect. In On the Origin of Species, Darwin attempted to preempt his critics by writing, "The crust of the Earth is a vast museum, but natural collections have been imperfectly kept."

One hundred and fifty years later, we acknowledge that the fossil record is still far from complete. Researchers studying fossilization and observing how dead animals decay over time have replicated the chemistry that turns living tissue into rock. Thanks to their work, we understand that other organisms quickly eat or decompose most dead animals. Very few living things are covered by sediment shortly after death, an essential step in the fossilization process. In addition, very few existing fossils are identified by human beings. Therefore, the fossil record is scarce. However, the few fossils we do have are strong evidence for evolution.

Over time, more fossils will be discovered, and other scientific disciplines will continue to enrich our understanding of evolution with additional lines of evidence. The puzzle will be slowly completed. Evolutionary theory will be modified along the way, but there is no doubt that we currently have an accurate picture of how life's history on Earth unfolded.