The world tree is present in various mythologies and religions, seen most recently here in my post about Valhalla, and is a symbol that connects the heavens, world and the underworld. It could perhaps also symbolise the tree of life that appears in religions that do not hold the same idea of a world tree.
Aside from Yggdrasil in Norse mythology as mentioned in my previous post, similar world trees that support the heavens are seen in Hungarian mythology (világfa), Turkic mythology (Ağaç Ana), Mongolian mythology (Modun), Germanic mythology (Irminsul), Slavic and Finnish mythology (the Oak), and Hindu mythology (Ashvattha).
World trees also appear throughout Mesoamerica, illustrations of them left upon the ruins of Palenque, for example, which show how world trees embodied the four cardinal directions, and hence the fourfold nature of a central world tree that connects the planes of the Underworld and the sky with the world. (My post regarding Mesoamerica explains how the Maya, Aztec, and others believed in 13 levels of heaven and 9 levels of the underworld that each have a wheel-like structure and interact with each other through this.)
The Maya tree, for example, is represented by a ceiba tree and is said to have birds in their branches. The yggdrasil in comparison is said to be an ash tree and is regarded as holy; various creatures, some mythological, also live within it. The Ashvattha (Hindu mythology) is a fig tree.
Despite this belief, some scholars suggest that the concept of the world tree has evolutionary origins because as we were once primates we spent a lot of time in trees, hence the ‘world as a tree’ is engrained in our collective unconscious. Even in religions and mythologies that do not believe in a world tree, a tree is often a central image within it, hence the link of the world tree with the tree of life, or the tree of knowledge – the Rastafarians, for example, consider cannibis to be the tree of knowledge, hence its use in meditation!