In a previous post I tried to define life, and failed a bit. Another interesting question, actually probably a much more interesting questions is; what different forms can life take?
Life on Earth typically comes in six main varieties; animals, plants, fungi, protists, bacteria and archaea. You know plants and animals already, and fungi are basically mushrooms and their relatives. These types of life have cells with nuclei (called eukaryotic cells) and are almost always multi-cellular organisms, they’re made-up of lots of different specialised cells working together. Protists are usually single-celled organisms, with eukaryotic cells; the most famous is the amoeba. Bacteria and archaea are also single-celled, but have less-complex cells without a differentiated nucleus called prokaryotic cells. Bacteria and archaea are similar in size and shape, but differ in terms of biochemistry. Nothing we need to go into here, but archaea are more likely to be extremophiles, more later.
Although there are six different varieties, life on Earth is extremely similar. A mushroom might look very different from a tiger, but they’re made from the same materials. Life on Earth is primarily made from carbon, oxygen, hydrogen, nitrogen and smaller amounts of around 25 other elements. These elements are arranged in molecules like proteins, fats, sugars and DNA. These molecules are then arranged in cells, essentially little bags of molecules undergoing controlled reactions. These cells are then combined in different ways to create different structures; from livers to bones, and from mushrooms to tigers. It’s a bit like Lego, there aren’t that many different pieces, but you can arrange them in different ways and different amounts to create an almost infinite number of structures. This is the same principle on Earth; it’s often quoted in biology textbooks as something like ‘diversity from uniformity’.
Life on Earth is typically called ‘carbon-based life’, as carbon is the atom that binds most of our molecules together. Think of carbon as a really useful Lego brick that can join to all other Lego bricks. Water is also a really important substance for life on Earth. Our bodies transport and react lots of different molecules to release energy and to create and repair important structures, and water is great at absorbing and transporting molecules, thus is often called ‘the medium of life’. This is why life, at least on Earth, contains a lot of water.
So life on Earth is very similar at a cellular and molecular level, and is based primarily on carbon and water. A bit boring? I’d say no. Scientists, and even regular people, are constantly finding new and bizarre forms of life in unexpected places; just watch any documentary on the deep oceans for weird examples. Some of the coolest are called extremophiles; organisms which thrive in extreme conditions, like living in acid or living in high-temperature locations. They’re almost exclusively archaea or bacteria and some notable examples include Pyrococcus furiosus which likes to live in 100°C water and Deinococcus radiodurans which has been named as the world’s toughest bacterium by the Guinness Book of Records as it can live comfortably with high-levels of radiation, acid, cold and dehydration. It can even survive in a vacuum for a time. Many forms of extremophile have been found deep underground in subsurface water or even in deposits of oil. Some scientists have even proposed that potentially more life may exist in the Earth’s crust than on the surface. So life on Earth isn’t dull by any stretch of the imagination.
But still, even though life on Earth comes in many varieties, it’s still very similar in terms of what it’s made out of, it still uses the same 30 or so Lego blocks. But a key topic in astrobiology is the consideration and search for other kinds of life, such as life that uses different Lego blocks, or doesn’t even use Lego at all. Such life may be found on different worlds, or may even exist on Earth.
Life on Earth is likely so similar because it all originates from a common ancestor; offspring have descended from this ancestor and have evolved into different species via the processes of natural selection. But what if there was a separate branch of life that evolved from a different ancestor, or split-off very early in the history of life? Such a potential life-form may have been discovered recently. NASA has published a paper in which they report to have found a bacterium that uses arsenic rather than phosphorous in its biological molecules. The debate is raging about whether this really is the case, but if so, this would be an example of life using one different Lego block.
But what about life using more alternative Lego bricks? Scientists have hypothesised that alien life could use different atoms. Silicon has been proposed as an alternative to carbon, as it can form large molecules like those used in our bodies, but its not quite as versatile as carbon and can’t bond with some of the important atoms used in life chemistry. Alternative mediums to water have also been theorized, including ammonia and methane. But these have problems too; ammonia can absorb molecules like water, but is liquid at a low temperature and thus biochemical reactions would occur slowly and life would be sluggish if not impossible. Methane has a similar problem with the cold, and is less capable of absorbing molecules than water. It may be that the laws of chemistry constrain possible life forms, and that carbon and water may be the best, and only solution.
But what about life that isn’t made of Lego? Could life exist which isn’t even based on atoms? Most ideas for such life-forms have come from science-fiction rather than science. But some very intelligent authors have used quite convincing rationales, so they shouldn’t be causally dismissed. The best examples I’ve seen are from Stephen Baxter, they include life-forms made from quarks (little bits of Lego bricks), life composed of fractures in space-time, black hole ecosystems, and the photino-birds, who are dark matter life forms. This all sounds fantastic, but over the course of his Xeelee novels Baxter provides compelling rationales for the existence of all of his species. In Baxter’s universe, our Galaxy is teaming with life, but very strange life. It’s exciting to imagine such possibilities.
I’ll probably dedicate future posts to looking at alternatives to carbon and water, and looking at more extreme ideas of life, as these topics are fun and really deserve to be explored in more detail.
Even if alien life is composed of similar building materials to us, I’m sure, if it exists, it will be outside the scope of our imagination. And if more exotic forms of life do exist, Baxterian life we should call it, then who knows what would be the limits on their bizarreness.