Life on Earth began about 3.8 billion years ago when our planet was a ball of molten rock, when the first organisms lived underground, using volcanic gases like hydrogen, iron, nickel, and carbon monoxide for energy instead of sunlight. From these early conditions, a single population called the Last Universal Common Ancestor (LUCA) evolved, grew, and reproduced, passing on about 355 genes that we can still trace in living organisms today. Over time, endosymbiosis occurred, where a bacterium engulfed another organism that became the mitochondrion; because mitochondria have their own DNA separate from the host, which is the bacterium, and this DNA is still passed on when the bacteria split, it shows how new. As Earth cooled and formed a solid crust, life moved to the surface, and through billions of years of evolution and changing conditions, that original life evolved into the many species we see today. We know this evolutionary history is real because we can trace LUCA’s genes back to the Earth’s crust. We also use phylogenetics to compare DNA and traits to see how species are related, and examine fossils and rock layers to find physical evidence of ancient organisms and environments. Phylogenetic trees organize this evidence into branching diagrams based on shared traits and genetics, showing which species share more recent common ancestors. For example, a phylogenetic tree of a gecko, a fungus, and a palm tree shows that the gecko and fungus are more closely related to each other than either is to the palm tree, even though they look very different, because their branches share a more recent common ancestor. Branches that split off earlier in a tree represent more distant relatives, while branches near the top show species that are more recent or still alive, which is how we can reconstruct how all living things are connected through common ancestry.
