Science-fiction television shows and movies often depict aliens as looking very much like humans. Discuss the likelihood that intelligent creatures from another world would have (a) a biochemistry similar to our own, (b) two legs and two arms, and (c) about the same dimensions as a human.

Exploring the biochemistry of alien life taps into the imaginative and the scientific alike. So why do we often imagine extraterrestrial life with a biochemistry similar to ours? This is partially because carbon, the backbone of all known life on Earth, is adept at forming stable molecules with many different elements, creating the diversity necessary for life.

Water, being a universal solvent with unique properties such as high heat capacity and surface tension, plays a fundamental role in Earth’s biochemistry, assisting reactions necessary for life. It's enticing to speculate that these same properties would be beneficial for life elsewhere. However, the universe is vast and varied, placing no limits on the possible solvents for life, like ammonia or methane, or even alternative biochemistries, such as silicon-based life, given silicon's chemical similarity to carbon.

Considering the probabilistic nature of the universe, the appearance of life with an entirely different biochemistry is not beyond the realm of possibility. Our exploration of this concept can shape our search for extraterrestrial life, guiding us to consider environments quite different from our own watery, carbon-based sphere.

The concept of bipedalism in extraterrestrial organisms stems from the observation that many creatures on Earth, including humans, have evolved to walk on two legs. This adaptation offers several advantages, such as freeing the hands for tool use and improving the field of vision.

However, the evolution of bipedalism relies heavily on environmental factors. If an alien planet has terrain and ecological conditions similar to Earth, we could see evolution promoting organisms with two-legged locomotion. But the gravity, climate, and available resources of an extraterrestrial world might lead to entirely different adaptations.

Life forms may evolve multiple limbs for stability or none at all, adapting to locomotion in their unique habitats. Therefore, while bipedalism is a known successful strategy on Earth, it is just one of many potential evolutionary solutions to the challenges of an organism's environment. Our understanding of evolution hints at an incredible diversity waiting to be discovered, potentially defying our bipedally-biased expectations.

When discussing the evolution of alien species, we must consider that evolution by natural selection is a universal principle, assuming life exists elsewhere. On Earth, organisms have evolved through natural selection to fill a myriad of ecological niches with various forms and functions.

The size and shape of any creature, human or alien, are influenced by numerous factors including gravity, atmospheric conditions, available nutrients, and the presence of predators or competitors. An alien species would be subject to similar pressures but shaped by their unique environment. If the conditions of an extraterrestrial planet mirror those of Earth, we might find creatures with resemblances to terrestrial forms.

On worlds with thicker atmospheres, life might evolve to be smaller or more robust to cope with increased atmospheric pressure. Organisms on low-gravity planets could be taller and lighter. This vast array of possibilities reminds us that the universe’s capacity for diversity is as unlimited as natural selection is creative. The evolutionary paths of alien species are an enigma we are only beginning to imagine.