Some like it hot . . . or cold
Life – always resilient – could still find ways to persist on planets that spin on their "sides," Uranus-style. Maybe migrating critters could follow a survivable, fast-shifting climatic zone while others find refuge at the equator. Hardy organisms might just ride out the temperature extremes. Examples of these rugged creatures right here on Earth, mostly bacteria, are known naturally enough as "extremophiles."
A class of these organisms, called thermophiles, thrives in hot springs and in the lightless oceanic depths around hydrothermal vents. The species Methanopyrus kandlerican reproduce in high-pressure waters hotter than 250 degrees Fahrenheit. On the other end, psychrophiles grow in ice-covered cavities of briny seawater down to 5 degrees Fahrenheit.
When conditions get too hot or cold, sporulating bacteria go into stasis, encasing themselves in tough structures called endospores. The microorganisms can lie dormant in ice for millions of years and upon thawing go right back to replicating.
Earth becomes more Earth-like
For more than the simplest biota, such feats of durability would surely pose a lot of challenges on exoplanets with higher obliquity than Earth's, though far less than that of Uranus.
"Perhaps an obliquity of just 40 degrees would be tough for complex animals due to the very hot summers and cold winters that would affect much of the globe," noted George Williams, a geologist at the University of Adelaide in Australia. [Extremophiles: World's Weirdest Life]
Climatic conditions of this very sort might have held back the evolution of big and diverse creatures on our planet, Williams suggests. Prior to about 580 million years ago, scientists think most of earthly life consisted of microscopic algae and bacteria. Complex animals such as jellyfish and worms arrived on the scene thereafter. Then, starting about 540 million years ago, in what is known as the Cambrian explosion, life went nuts. All sorts of intricate body types sporting spines, shells, eyes, legs and more suddenly show up in the fossil record.
Could Earth have once possessed a high obliquity? Computer models say yes. The cataclysmic impact with a Mars-sized body 4.5 billion years ago, thought to have created the moon, could have knocked Earth's spin axis well off-kilter from the plane of the planet's orbit. Intriguingly, some geological evidence is consistent with Earth having a high obliquity for much of its history, up until about 600 million years ago.
Glaciers provide crucial information in this regard. As shown by numerous geophysicists headed by Phil Schmidt at the Commonwealth Scientific and Industrial Research Organisation in Australia, glaciers used to form preferentially in formerly low latitudes. (Somewhat counterintuitively, an obliquity exceeding 54 degrees renders the equator cooler than the poles, on average.)
Magnetic directions fixed in glacial deposits have revealed this ancient icy activity. Associated sand-wedge structures, like those that occur in modern-day polar regions, suggest big seasonal temperature fluctuations as well. From winter to summer near the former equator, temperatures varied in excess of 100 degrees Fahrenheit. If that were to occur today, blizzards could dump snow on the Amazon rainforest.
Geological markers of high obliquity peter out around the so-called Precambrian–Phanerozoic boundary. After this time, significant glaciations occurred in just the high latitudes, and life took off.