Stars, Life, and Sight

Exoplanets have quickly become the focus in the search for life beyond Earth and the quest to know whether we are alone in the unfathomable depths of space. Finding a large number of habit­able exoplanets would suggest that life abounds in the universe. For now, astronomers are scouring stars in the Milky Way most likely to host habitable worlds. Stars range in size from small, cool dwarfs no bigger than Jupiter to colossal stars topping out at about 200 solar masses and 10 million times brighter than our Sun. Blue giant stars are very hot, bright suns, such as those in the Pleiades Cluster or in Orion’s Belt, that emit mostly UV light. About half of all stars are binaries, in which one star orbits another. Binary

star systems can host planets with stable orbits around both stars, called circumbinary planets (figure 9.3). The first of such exotic exoplanets, reported in 2011, was dubbed the Tatooine planet for its dual sunsets as depicted in the film Star Wars.15 But in some binary systems gas interactions between the stars produce intense X-ray radiation deadly to life on their attendant planets. The most common stars in the Milky Way galaxy are M dwarfs. With less than half the mass of our Sun, M dwarfs emit most of their light at infrared wavelengths. Brown dwarfs, too small to generate nuclear fusion in their core, emit light solely in infrared. Among the many types of stars, it seems we couldn’t have wished for a better star to orbit, nor one more suited for life, than our Sun.

Any type of star can host planets, but the light stars pump out determines whether, and what kind of, life might survive on their

satellite worlds. Astronomers Ray Wolstencroft and John Raven contend that life works most efficiently with blue light, which they argue triggered photosynthesis on Earth.26 If that’s true, one might assume that planets orbiting hot, blue stars would be the most likely to host organisms. However, large blue stars emit dangerous ultraviolet radiation and expend their fuel in a mere 10-20 million years. By contrast, our Sun has existed for 4.5 billion years, and astronomers expect the Sun will not expend its fuel for another 4 billion. It’s our Sun’s long life span, in part, that allowed time for photosynthesizing organisms to emerge. As it happens, our Sun is the type of main sequence star that emits its greatest amount of radiation in the range of visible light. At some point 3 billion years ago, living organisms began to harness this light for processes in photosynthesis.27 Cyanobacteria or its progenitor began to use sunlight and carbon dioxide to generate food in the form of sugars, and in turn release a tiny gulp of oxygen. That seemingly simple development slowly, but radically, began to alter the chemistry of Earth’s atmosphere from one conducive to anaerobic life to an atmosphere rich in oxygen. But oxygen levels sufficient to sustain complex life as we know it didn’t evolve until approximately 550 million years ago. As Leslie Mullen reports, “If the evolution of photosynthesis is the same everywhere, then the lifetime of blue stars is just not long enough” for life to emerge on surrounding planets.28

Though no one knows for sure exactly how life emerged, a group of scientists at the Santa Fe Institute posit that the reductive Krebs cycle may have brought together chains of carbon molecules to produce the first organisms.29 The Krebs cycle, or citric acid cycle, describes how organisms produce energy by breaking down carbohydrates, fats, and proteins. In what has been called the “me­tabolism first” theory, the reductive Krebs cycle is proposed as the mechanism through which the first organisms emerged. “In some primitive single-celled organisms, [the Krebs cycle] operates in re­verse—it takes in energy and puts small molecules together into big ones. Scientists term the reactions in this mode as ‘the reductive Krebs cycle.’”30 Eric Smith, a professor at the Santa Fe Institute, has argued that geochemical processes on the early Earth might have combined smaller molecules into larger ones because it was energetically favorable to do so, similar to the chemical reactions in the reductive Krebs cycle. In other words, life might be built into, or be an expected outcome of, terrestrial geology.

Plants stretch their stems and leaves toward the Sun, and even the earliest organisms would have used a single photon of light for fuel and to obtain valuable information about the immediate envi­ronment. “The very pigments that led to photosynthesis probably also allowed those earliest creatures to react to light,” writes Ben Bova, who notes that even “single-celled organisms have light re­ceptors, sensitive spots of pigment that absorb light.” Michael So – bel’s book Light, Michael Gross’s Light and Life, and Bova’s The Story of Light explore the myriad ways life on Earth is enabled by, and is adapted to, the light of our Sun. Photoreceptor cells in bacteria, plants, and animals demonstrate the selective pressure of visible light, as such cells evolved due to the survival advantage they provided organisms on a planet with a star that emitted its greatest radiation in visible light. “The very earliest creatures cer­tainly did not have a sense of vision,” notes Bova. “All they could do was react to light the way a scrap of paper reacts to a puff of wind—move either toward the light or away from it.”31

By the Precambrian period, soft-bodied marine animals emerged that had at least a rudimentary sense of light and darkness. Under such conditions, finding prey and avoiding being eaten were dif­ficult prospects. But life on Earth would suddenly, on geological timescales, change. Neurobiologist Michael Land and zoologist Dan-Eric Nilsson note that “something remarkable seems to have happened in the interface between the Precambrian and the Cam­brian eras. Within less than five million years, a rich fauna of mac­roscopic animals evolved, and many of them had large eyes.”32 Land and Nilsson posit that a simple strip of photoreceptive cells in organisms could, in time, become curved to better sense the ori­entation of the incoming light. If that curved space became filled with a liquid, the light could be focused on a lens to create image­forming vision. Nilsson and Land conservatively estimate that the evolution of image-forming eyes could occur in a half million years, and perhaps as little as 300,000-400,000 years.

Land and Nilsson, as well as zoologist Andrew Parker, have re­searched how eyes may have emerged in response to the light pro­duced by our Sun. The first sophisticated eyes, dating to 530 mil­lion years ago, appeared at the beginning of the Cambrian period and they’re evident in the extraordinarily well-preserved fossils of the Canadian Burgess Shale. Parker refers to the Cambrian as the “big bang” in evolution: “It paved the way for the emergence of the vast diversity of life found today, whether in Australia’s Great Barrier Reef or Brazil’s tropical rainforests. It involved a burst of creativity, like nothing before or since, in which the blueprints for the external parts of today’s animals were mapped out. Animals with teeth and tentacles and claws and jaws suddenly appeared.”33 Parker argues that visible light acted as a strong selection pres­sure for the development of eyes in most species. In what he calls the “Light Switch” theory, Parker contends that sunlight has been the primary factor in determining animal evolution, behavior, and morphology. With the emergence of more complex eyes, Cambrian species could colonize niches within the water column of ancient oceans that in turn impacted their morphology. Trilobites, for instance, thrived in Cambrian seas (there were some 4,000 spe­cies) and apparently were the first animals to develop compound eyes. The trilobite could clearly see its environment, navigate, and seek prey. As a result, they and many other species developed exo­skeletons to protect themselves. Coincident with the evolution of eyes that could focus, Parker argues, the fossil record reveals the emergence of a radical diversity of all phyla of animals. Land and Nilsson agree: “If eyes had not evolved, life on earth would have come out very differently. More than any other organs, eyes have shaped the evolution of animals and ecosystems since the Cam­brian explosion.”34