The inquiry into where the Earth begins and ends has captivated humanity throughout history. Many ancient creation myths describe the Earth with distinct borders; for instance, Mesopotamian tales depict it as a flat disk afloat in an ocean, encircled by a dome of sky. The Hopi tribe imagines a series of layers, from which humans have ascended into the fourth layer, escaping the chaos below via a hollow reed in the Grand Canyon. The Greeks were pioneers in conceptualizing Earth as a sphere, devoid of definitive edges.

Over time, scientific advancements and explorations have transformed our understanding of frontiers, revealing them to be fluid and uncertain. These boundaries serve as both limits and gateways, marking the transition between the known and the unknown. They define territories yet also allow passage into unfamiliar realms, altering the very nature of our experience.

The Greeks often relied on celestial observations to support their hypothesis of a spherical Earth. Aristotle noted that the Earth cast a circular shadow on the moon during lunar eclipses, while constellations appeared differently as one traveled southward. Another line of reasoning drew from practical observations, particularly concerning elephants in Alexander the Great’s campaigns. The Greeks surmised that the presence of elephants in various directions suggested a world enveloped by these majestic creatures.

Medieval world maps, or mappae mundi, provided both guidance and wonder, portraying possibilities beyond distant frontiers. This perception of a round Earth influenced Greek views on exploration; unknown lands were seen as attainable rather than unreachable. Fast forward to today, as our planet is thoroughly documented, the new frontier appears to lie in outer space. Yet, the potential for internal transformation—conceptual and moral—may hold greater significance for our future.

In the wake of the fall of classical civilizations, Europeans redefined their world. They crafted intricate maps that reflected a divine order rather than empirical investigation. This worldview persisted for over a millennium, as illustrated by a notable map from around 1300 A.D. housed in Hereford Cathedral, depicting a round but flat Earth centered on Jerusalem. The map distorts Europe while portraying fantastical creatures and hybrids in unexplored territories.

These maps, akin to medieval bestiaries, offered both wonder and caution. They revealed alternatives beyond distant frontiers while warning of the dangers of curiosity. St. Augustine condemned such inquisitiveness as concupiscentia oculorum, or lust of the eyes.

The exploration of our fellow creatures' inner lives presents an expansive frontier of beauty and wonder that reshapes our understanding of our own boundaries. The 15th century marked the beginning of a transformative intellectual journey. In 1417, Poggio Bracciolini unearthed Lucretius's De Rerum Natura, which introduced the revolutionary concept of atomic composition. This revival of ancient wisdom, coupled with accounts from adventurous travelers, fostered a burgeoning recognition of humanity's potential.

The Erdapfel, or Earth apple, symbolizes this shift. Crafted in Nuremberg in 1492, this terrestrial globe showcased knowledge previously unknown to the ancients, depicting the African coast and the route around the Cape. It emphasized practical navigation over mythical creatures, reflecting the aspirations of the Portuguese monarchy.

Despite the absence of the Americas on the Erdapfel, its inaccuracies reveal a newfound confidence in humanity's ability to traverse the globe and uncover its treasures. However, this era of exploration was not without its darker aspects. The empires of early modern Europe were ruthless, and figures like Francis Bacon sought results over wonder, viewing the latter as “broken knowledge.”

Yet, the scientific age suggests that increased knowledge does not diminish wonder; rather, discoveries reveal complexities more astonishing than the mysteries they solve. Geologists of the 19th century unveiled the concept of "deep time," illuminating the vastness of prehistory and shifting humanity's perception of time and existence. John Playfair remarked on the dizzying realization of this new temporal dimension, where ancient life thrived long before humans.

Charles Darwin's theory of natural selection, articulated in On the Origin of Species, elegantly followed from this understanding of deep time, challenging long-held beliefs about species' immutability. His insights illustrated life's remarkable adaptability and interconnectedness with nature, ultimately redefining humanity's place within it.

Darwin's perspective was both awe-inspiring and brutal, depicting life as a relentless struggle for survival. Yet, it concluded on a note of grandeur, positing that from simple beginnings emerge endless forms of beauty. His findings prompted a reevaluation of humanity's origins, suggesting a continuity with the animal kingdom, though acceptance of this idea was not universal.

The exploration of animal cognition and emotion has since gained traction, challenging previously held beliefs about human uniqueness. Recent studies affirm that traits once thought exclusive to humans, like self-awareness and emotional depth, are present in various species, prompting a reevaluation of our understanding of consciousness.

The microbial realm offers yet another frontier of discovery. Antonie van Leeuwenhoek's 17th-century observations introduced us to an unseen world, revealing the complexity of microorganisms. Lynn Margulis's theory of endosymbiosis, which posits that complex life forms evolved from simpler organisms merging, reshapes our understanding of life’s interconnectedness.

As our comprehension of microbiomes and their influence on human health deepens, we begin to question where humanity ends and microorganisms begin. This intricate relationship challenges the boundaries we impose between ourselves and other life forms.

In the depths of the ocean, microbial life thrives in extreme conditions, reshaping our understanding of life's potential. Discoveries in Antarctica reveal life flourishing in environments previously deemed inhospitable, prompting us to reconsider the limits of where life can exist.

As we turn our gaze to the cosmos, the possibility of extraterrestrial life becomes increasingly plausible. The advent of advanced telescopes may soon allow us to detect biosignatures on distant planets, igniting curiosity about what forms of life might exist beyond Earth.

In our current epoch, characterized by rapid environmental change, humanity faces a moral frontier. The Anthropocene highlights our profound impact on the planet, raising questions about our responsibility and influence. While some argue that our actions reflect control, we must acknowledge that influence does not equate to mastery.

The unpredictable nature of ecological shifts necessitates a reevaluation of our role within the biosphere. Historical precedents remind us of the catastrophic consequences of environmental strain, urging caution as we navigate the complexities of modern existence.

While technological advancements present opportunities for innovation, they also raise ethical dilemmas. The possibility of de-extinction and genetic manipulation invites scrutiny of our intentions and the ecological contexts in which these actions occur.

As we reflect on the past, we recognize that our history is intertwined with the fate of other species. The decline of elephants serves as a reminder of the delicate balance between coexistence and dominance.

Ultimately, our understanding of boundaries—both natural and conceptual—continues to evolve. We exist at a precipice, capable of fostering innovation while remaining mindful of the ethical implications of our actions. In the words of Italo Calvino, we must seek to recognize and nurture the aspects of existence that transcend the chaos around us, embracing the potential for beauty and connection.

Caspar Henderson is the author of The Book of Barely Imagined Beings: A 21st Century Bestiary (2013, Chicago University Press).