The great development of the brain is associated with the stone age, when available materials began to be used as tools, and this use engendered the development of thinking and the evolution of larger brains. As far as we know, most planets like Earth and originating life will have available stone resources, so there is little reason to assume the lack of stone will be a barrier to the development of an alien civilization. The same goes for other items that could be used as improvised tools, such as sticks, hides, leaves, reeds, vines, and certainly others. Stone is the only one which lasts for a million years, so we have the ‘Stone Age’ in our pre-history books, instead of the ‘Stone and Stick and Vine and Other Stuff Age’, but that is the expected situation, lasting for a million years or two or maybe something a bit shorter or longer on other planets.
Gold is the only metal found predominantly in the metal form. Some copper is also, but most copper along with other early metals have to obtained from their ores by roasting, at the very least. This is the simplest form of smelting. The use of fire is necessary, and so the question is, how did metals become discovered? Clay was used on Earth around the same time, and clay can be dried to transform it into something waterproof and rigid, but heating it makes many clays even stronger, sometimes producing a coating on their exterior. That means hot fires are needed. Even earlier than this, certain stones were thermally fractured, notably flint, so in this situation we have a culture that is used to putting rocks into a hot fire, maybe in a pit, and seeing if they fractured into sharp edges. Putting rocks into a clay kiln appears much less likely, but certainly possible, perhaps to support the fuel.
The earliest metal droplets found in archaeological digs were tin and lead, although this may be affected by chance finds. Copper is the metal that makes all the difference. Roasting copper ores with charcoal can produce copper, which is a malleable metal. Some copper ores contain arsenic, which makes the metal much harder, which would be preferred for some uses, such as weapons. Arsenical bronzes, which are just copper with the major impurity being arsenic, were invented at least twice, in the Andes and in Asia minor, and possibly many other places as well.
These metals might seem a mandatory material for an alien civilization to proceed out of the stone age, but cultures which did not have them seemed to be able to produce all the trappings of a civilization without them. The Mayan states had no metal, but they had monumental cities, writing, a religious hierarchy, organized agriculture, craftsmen, paints, and other accouterments of civilization. They even invented rubber balls and ball games, which the Eurasians never did. But the Mayan civilization could not progress beyond the agricultural grand transition without metal. It is simply not possible to move to energy sources beyond animal power without metal, and energy is one of the inputs to an industrial civilization. Even animal power is difficult to make the best use of without metal. So, copper ore and in fact, metal ores for many other metals are mandatory surface items for civilization to develop.
To have copper ore on the surface of an exo-planet of roughly Earth size requires that there was copper in the initial gas cloud which condensed into the planet, and that there was a segregation of elements in the molten magma, leading to lighter ores which could rise to the crust and condense. Then they have to be exposed.
In order for life to form, there must be some minerals, but they can be principally phosphorus, magnesium and other lighter elements. Microbes do not need copper to exist and evolve, bu there are some uses for it. This implies that a planet can originate life without heavier metals, but some efficiency in chemical processing would be lost if they are absent. How far evolution could go on a planet where there were no or few sources of heavier metals is not clear, as we do not understand anything about alternative forms of life. However, if it is possible to evolve land animals without metals like copper, civilization would likely be stopped by the lack of metals at the agricultural stage.
There does not seem to be any reason why an alien civilization would not discover smelting of at least bronzes, just by chance. The Mayan example means that it takes time for the right accident to happen, or possibly that if smelting were discovered accidentally, something in the culture, like a theocratic decision, might have forbidden a continuation of the discovery.
After a culture has been in the bronze age for a period, iron is usually discovered, as we have seen in different regions on Earth. But iron ore must be heated to higher temperatures than copper ore if metal is to be extracted. Charcoal can produce such temperatures in the right type of vessel, but there are some requirements. Air does two things: it provides more oxygen for more combustion in the same region of fuel, and it cools the fuel by conducting heat away from the combusting surfaces. Clearly, the fraction of air that is oxygen controls whether the temperature generated by the combustion reaches the temperature needed. A planet with a lower fraction of oxygen would have lower temperatures in the same physical arrangement, and if the fraction were sufficiently low, would not be able to smelt iron ore.
Similarly, if the atmospheric pressure were less, oxygen would be delivered by convection to the burning fuel more slowly, and again, the temperatures generated would be less. This may help explain why the Inca nation, principally living at higher altitudes in the Andes, never made the leap from copper alloys to iron. Temperatures in most of their kilns were lower, and only the smaller number of low altitude regions could possibly do it, reducing the probability of the discovery of iron as a more useful metal.
Thus, if oxygen content and atmospheric pressure are either too low on an origin planet, the civilization that might develop there would not make the transition into an industrial era, not having iron or perhaps, if one of these two were significantly lower, bronze. Some more thinking into what controls these two quantities might be useful in helping us locate potential worlds with other civilizations.
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