The discovery of a new species of harvestman in Sri Lanka, as detailed in the paper by Prashant Sharma and Gonzalo Giribet, provides a fascinating glimpse into the biodiversity of these small, leaf-litter dwelling arachnids. The authors describe Pettalus lampetides and, more significantly, document the presence of eyes in a family, Pettalidae, long thought to be entirely blind. Based on this finding, the paper makes a broader evolutionary claim: that the common ancestor of the entire Cyphophthalmi suborder likely possessed eyes, and that subsequent eyelessness in many lineages is the result of repeated loss. This argument, however, expertly maps a pattern of existing traits but fails to provide any evidence for the unguided mechanisms required to originate those traits. The study presents a mosaic of variations on a pre-existing body plan, framing these different implementations as evidence for a creative process, when the more fundamental question—the origin of the plan and its complex systems—is left unaddressed and unproven.
Critical Analysis
The Finding: Eyes Discovered in a “Blind” Family (Direct Evidence)
The paper’s central discovery is the presence of simple eyes in Pettalus lampetides and, upon re-examination, in other related species within the Pettalidae family. For decades, this family, with one known exception, was characterized as blind. The authors use microscopy to show that these eyes are clearly present, located at the base of the creatures’ defensive scent glands, or ozophores.
The Evolutionary Counter-Argument:
Proponents of the grand evolutionary narrative will state that this discovery clarifies the ancestral state of this group of organisms. By applying the principle of parsimony, finding eyes in this branch of the harvestman family tree makes it most likely that the common ancestor possessed eyes. Consequently, blindness is not an ancestral state but a derived trait that appeared independently in multiple lineages through evolutionary loss.
The Rebuttal:
This line of reasoning confuses pattern with process. Mapping the presence or absence of a feature onto a phylogenetic tree is a descriptive cataloging exercise, not an explanation of cause. It reveals a pattern of inheritance and loss, but is entirely silent on the biogenesis of the feature itself. The analysis demonstrates that the genetic information for building eyes was likely present in the ancestor and subsequently lost or silenced in certain descendants. This is a story of information loss, a common and expected outcome of random mutation. It provides no evidence whatsoever for the de novo generation of the vast, specified information required to construct a functional eye system, however simple.
The Finding: Varied Integration of the Eye System (Direct Evidence)
The paper notes a key difference in how these eyes are integrated into the animals’ bodies. In the newly described Pettalus, the eyes are physically incorporated into the base of the ozophores. In another family, Stylocellidae, the eyes are located separate from and anterior to the ozophores.
The Evolutionary Counter-Argument:
This variation in the placement and integration of a shared organ is often presented as a classic sign of unguided evolutionary tinkering. The argument is that evolution works with what it has, modifying and repositioning pre-existing parts in a step-by-step fashion, leading to different but functional outcomes in different lineages.
The Rebuttal:
Observing minor variations in the “packaging” of a complex module does not explain the origin of the module. Consider two different models of a vehicle, both of which use the same complex engine. One may be a front-engine design and the other mid-engine. An engineer can study the different mounting brackets, drive shafts, and chassis reinforcements, but this analysis says nothing about how the internal combustion engine itself—a highly integrated system of pistons, cylinders, and electronics—came to be. Similarly, finding that the harvestman eye is mounted in slightly different locations relative to the ozophore offers no insight into the origin of the eye’s own integrated complexity: its lens, photoreceptors, and neural connections. This is evidence of limited modular adaptation within a pre-existing architecture, not the construction of the architecture itself.
The Bigger Picture
This paper exemplifies a widespread tendency in evolutionary biology to conflate the history of a trait’s distribution with an explanation for its origin. The phylogenetic analysis, based on the parsimonious distribution of traits, is a tool for classification. It creates a plausible branching diagram of relationships based on similarities and differences. Yet, it does not possess the power to demonstrate the mechanism by which novel structures and the information to build them arose. The study focuses on low-level adaptation—the loss of a system or the minor repositioning of a component—while leaving the far more challenging problem of invention entirely untouched.
Broader Context
The approach seen here is not unique. From the radiation of finches in the Galapagos to the diversification of mammals on islands like Sri Lanka, evolutionary biology often focuses on variation within an established kind. Studies on Sri Lankan fauna, for example, document a rich diversity of endemic subspecies that are adapted to different climatic zones. These subspecies represent phenotypic variations that allow mammals to fill slightly different ecological niches. This is a powerful illustration of adaptation and diversification. However, these studies describe the partitioning of existing niches and the modification of existing genetic information, not the creation of new genera or the information required to build novel organ systems. Phylogenetics maps these patterns but inherently assumes the creative power of the underlying mechanism it is meant to be testing.
Bottom Line
Sharma and Giribet’s paper is a valuable contribution to arachnology, providing a meticulous description of a new species and correcting a long-standing misconception about the presence of eyes in the Pettalidae family. However, the evidence presented does not support the grander evolutionary claims that are layered on top of it. Documenting the distribution of a complex feature like an eye across different species, noting its presence, absence, or varied placement, is not the same as demonstrating its unguided origin. The evidence points to the inheritance, modification, and, most clearly, the loss of pre-existing, complex functional information—not its spontaneous generation.
Paper Details
Title: “A NEW PETTALUS SPECIES (OPILIONES, CYPHOPHTHALMI, PETTALIDAE) FROM SRI LANKA WITH A DISCUSSION ON THE EVOLUTION OF EYES IN CYPHOPHTHALMI”
Authors: Prashant P. Sharma and Gonzalo Giribet
Journal: The Journal of Arachnology, 2006