still prescribed in some countries as an effective treatment for leprosy and other illnesses. But thalidomideâs use by pregnant women is necessarily and strictly forbidden.
Handedness is a biological phenomenon, with proteins being almost exclusively on the left. The origin of this preference is not known. It might be as simple as a chance event, but one that stuck. The uniformity points toward the development of a system with a single point of origin. If life had evolved twice, the likelihood would be that we would see both left- and right-handed proteins.
DNA has a similar but opposite steadfast bias: it is always right-handed. Point the index finger on your right hand and draw an imaginary clockwise circle in the air. At the same time move your hand away from your body. That is the turn of the double helix in its most common form, like a typical wood screw. 9 Even though that mirror-image double helix could perfectly well exist, it just doesnât. Life, yet again betraying its unique origin, only uses right-handed DNA.
The fact that a mirror world could exist but doesnât shows forcefully that life is of singular origin. If there ever was a choice between left and right, only one was selected, and the other discarded, never to feature again in the natural course of things. At the beginning of these biological mechanics, it might be as simple as a flipped coin, a chance event that was followed with absolute fidelity forever.
The Singularity
The origin of cells from parent cells and the origin of species via slowly changing genes in those cells both bear the hallmarks of a single origin. Those three aspects of biologyâcells only from existing cells, DNA changing through imperfect copying, and modified descent of a species as a resultâlogically unveils a single line of ancestry that inevitably leads back to a single point in our deep, deep past.
In other words, any one of the new cells that were pulled from their parent as a result of the orchestrated disaster response to a paper cut bears a direct and noble lineage. Life is replete with extinction, but a new cell on your hand is one of the great survivors of the longest pedigree in history. In one generation its ancestry traces back to the fertilized egg from which every cell in your body was born. The egg and sperm that fused to form your unique combination of genes and DNA can trace a similar pathway back to their fertilized egg origin, and so on through their parents, grandparents, and every ancestor in your family treeâand, indeed, the history of our species.
Naturally, it doesnât nearly end there. These cells, in turn, trace their ancestry through the life span of our apelike ancestors. We donât know exactly who they were yet, but they existed in the bodies of individuals who first carved bones to make tools, started fires with flint, and stood upright as we, among apes, uniquely do. 10
And it certainly doesnât end there, either. Before being in the loins of that upright ape, your cell lineage was carried through long-gone primates, maybe something like
Proconsul
, which resembled something between a chimpanzee and a macaque. Before them, it lay in monkeys and earlier through their furry, wet-nosed ancestors, more like modern lemurs. And as we follow them back, at some point, cells that were once the predecessors of your Pac-Man macrophages or sparking neurons existed in a shrewlike creature with fur and nipples. That critter, your ancestor, was present when an enormous meteor thundered into the Tropic of Cancer and called time on the reign of the dinosaurs sixty-five million years ago. Your cell lineage witnessed the entire fall and rise of those beasts inside many forms of early mammals, such as the
Cynodontia
, which looked like a snarling rodent of unusual size. Before those first mammals appeared more than 220 million years ago, your cellâs ancestor was a shelled egg of a reptile, hairless and cold-blooded, such
Handedness is a biological phenomenon, with proteins being almost exclusively on the left. The origin of this preference is not known. It might be as simple as a chance event, but one that stuck. The uniformity points toward the development of a system with a single point of origin. If life had evolved twice, the likelihood would be that we would see both left- and right-handed proteins.
DNA has a similar but opposite steadfast bias: it is always right-handed. Point the index finger on your right hand and draw an imaginary clockwise circle in the air. At the same time move your hand away from your body. That is the turn of the double helix in its most common form, like a typical wood screw. 9 Even though that mirror-image double helix could perfectly well exist, it just doesnât. Life, yet again betraying its unique origin, only uses right-handed DNA.
The fact that a mirror world could exist but doesnât shows forcefully that life is of singular origin. If there ever was a choice between left and right, only one was selected, and the other discarded, never to feature again in the natural course of things. At the beginning of these biological mechanics, it might be as simple as a flipped coin, a chance event that was followed with absolute fidelity forever.
The Singularity
The origin of cells from parent cells and the origin of species via slowly changing genes in those cells both bear the hallmarks of a single origin. Those three aspects of biologyâcells only from existing cells, DNA changing through imperfect copying, and modified descent of a species as a resultâlogically unveils a single line of ancestry that inevitably leads back to a single point in our deep, deep past.
In other words, any one of the new cells that were pulled from their parent as a result of the orchestrated disaster response to a paper cut bears a direct and noble lineage. Life is replete with extinction, but a new cell on your hand is one of the great survivors of the longest pedigree in history. In one generation its ancestry traces back to the fertilized egg from which every cell in your body was born. The egg and sperm that fused to form your unique combination of genes and DNA can trace a similar pathway back to their fertilized egg origin, and so on through their parents, grandparents, and every ancestor in your family treeâand, indeed, the history of our species.
Naturally, it doesnât nearly end there. These cells, in turn, trace their ancestry through the life span of our apelike ancestors. We donât know exactly who they were yet, but they existed in the bodies of individuals who first carved bones to make tools, started fires with flint, and stood upright as we, among apes, uniquely do. 10
And it certainly doesnât end there, either. Before being in the loins of that upright ape, your cell lineage was carried through long-gone primates, maybe something like
Proconsul
, which resembled something between a chimpanzee and a macaque. Before them, it lay in monkeys and earlier through their furry, wet-nosed ancestors, more like modern lemurs. And as we follow them back, at some point, cells that were once the predecessors of your Pac-Man macrophages or sparking neurons existed in a shrewlike creature with fur and nipples. That critter, your ancestor, was present when an enormous meteor thundered into the Tropic of Cancer and called time on the reign of the dinosaurs sixty-five million years ago. Your cell lineage witnessed the entire fall and rise of those beasts inside many forms of early mammals, such as the
Cynodontia
, which looked like a snarling rodent of unusual size. Before those first mammals appeared more than 220 million years ago, your cellâs ancestor was a shelled egg of a reptile, hairless and cold-blooded, such
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