All life is connected, and this pendant represents the entirety of our Earth-bound biological extended family. A phylogenetic tree is a diagram that shows the evolutionary relationship between species. Every form of life on earth comes from common ancestry, which you can see as the tiny vertical line in the center of the pendant. Each branch extending outward represents a split in the evolutionary path. The little circle on the right side of the pendant shows where humans reside in the diagram... like a "You Are Here" dot marking our place in evolutionary history.
Although diagrams similar to phylogenetic trees had existed previously, Charles Darwin popularized the concept in The Origin of Species. They are still widely used because they so effectively communicate evolutionary backgrounds.
This necklace is available in polished steel or gold plated steel. The pendants come on a surgical steel or gold plated steel 18 inch chain. Neither the pendants nor the chain will tarnish, saving you the trouble of periodic polishing. With reasonable care, your necklace should last and bring you science joy for years to come.