living fossil
Refers to a species that has remained unchanged for millions of years and closely resembles fossils of its ancestors. Used both in biology and figuratively for something that seems ancient or unchanged.
The coelacanth is called a living fossil.
This old custom is truly a living fossil.
Compound of 生きた (ikita, 'living') and 化石 (kaseki, 'fossil'). The phrase is a direct translation of the English term 'living fossil', which was coined by Charles Darwin.