Hierarchical design, as seen in engineering projects, takes foresight. Neo-Darwinism has no foresight, as Marcos Eberlin explains in his book Foresight: How the Chemistry of Life Reveals Planning and Purpose. In Darwin’s Dilemma (2009), animation portrayed construction of a computer network from the circuit design up. This illustrated how body plans in Cambrian animals exhibited hierarchical design. Some recent scientific papers have called attention to hierarchies in biological designs. Are they able to Darwinize them?
Water lilies thrive on water because of the hierarchical design in the structure of their floating leaves. In the AAAS Journal Science Advances, Ning Xu et al. at Nanjing University in China1 describe how the leaf is constructed in layers that work together, then tell how the design gave them ideas:
Water lily naturally (Fig. 1A) has an elegant system of transpiration with several features. First, the upper epidermis absorbs sunlight and provides stomata for water vapor escape. Its well-known hydrophobic surface has self-cleaning property. Second, the water lily can float naturally on the surface of water because of the existence of lacunae (air chamber) at the bottom of the leaf. Third, there exists a confined water path provided by vascular bundles to pump up water and then spread it into the large surface of the water lily.
Taking advantage of these features, we propose a water lily–inspired hierarchical structure (WHS), which can realize highly efficient, stable solar evaporation in high-salinity brine/wastewater until complete separation of water and solute is obtained. [Emphasis added.]
Another Chinese team, at Peking University, meanwhile, studied a floating fern named Salvinia, and found that hierarchical structures gave it properties of super-repellency to water by creating a kind of “air mattress” that bobs the leaf to the surface. They also wrote about their findings in PNAS.2
Beyond superhydrophobicity, here we find that the floating fern, Salvinia molesta, has the superrepellent capability to efficiently replace the water in the microstructures with air and robustly recover the continuous air mattress. The hierarchical structures on the leaf surface are demonstrated to be crucial to the recovery.
The Chinese seem to be on a roll looking for hierarchical structures in organisms. At Shanghai Jiao Tong University, another team examined photonic structures that confer a brilliant reflective white sheen in animals as diverse as beetles, butterflies, aquatic organisms, and mammals. This sheen is important for thermal regulation as well as species recognition. Writing in PNAS,3 the team saw that this trait required hierarchical organization at the nanoscopic level.
Very recently, our group reported that the white beetle Goliathus goliatus with an exquisite shell/hollow cylinder structure and the silvery butterfly Curetis acuta with irregular scales exhibit broadband reflectivity and radiative heat dissipation. Many of these biological photonic structures have evolved to be hierarchical ones characterized by distinct sizes and dimensionalities of structural building blocks. The building blocks, including nanoholes, nanorods, and nanobridges, are sequentially or randomly patterned to form integrated systems.
Biological hierarchical structures often go beyond getting the materials and shapes right; they also optimize how they are interconnected. The American Chemical Society took a closer look at nacre, or mother-of-pearl, which consists of layers of mineral and protein. Chemists have tried to imitate its toughness, but so far have failed. One reason is that chemists were simply layering the materials like bricks. In the oyster shell, “the natural material has wavy bricks that interlock in intricate herringbone patterns,” they found. When the chemists imitated that, the strength of their mimic was multiplied fourfold. It became “almost” as strong and tough as natural nacre.
Meanwhile, in the Netherlands, scientists investigated hierarchical design in relationships between genes. Writing in Science Advances,4 they noticed that transcription factors for genes are not randomly distributed. The caption for Figure 4 reads:
On the basis of the proven functional interactions shown in Figs. 4 and 5. Notch signaling (indicated by the open arrow symbols) induces Tcf1 expression that subsequently has two target genes: Gata3 and Bcl11b. Gata3 has a minor role in supporting development along the T cell linage but mainly acts to suppress the myeloid and B cell fates. In contrast, Bcl11b induces a T cell–specific program but has minor roles in suppressing alternative lineages with exception of NK cell development that is suppressed by Bcl11b. Collectively, there is a clear functional hierarchy of transcription factors.
A Rube Goldberg device may be irreducibly complex in the sense that removing one part destroys function, but it may not represent the optimum way to get a job done. Should it really take twenty crazy steps to operate an automatic back scratcher? In human engineering, time-motion studies can determine the most efficient way to perform a task, using just-in-time delivery, route optimization, feedback regulation and other methods. Many biological systems have already mastered hierarchical dynamics. A good example is the mammalian nervous system. Teng et al. at the Beijing Academy of Sciences write in PNAS5:
The mammalian nervous system, as the most powerful natural ionic circuitry, demonstrates specific functions, such as multiple responsive abilities and high-throughput ionic information transmission. The natural system employs ions rather than electrons as its carriers, due to the aqueous nature of the organism. Within this aqueous ionic system, billions of stimulus–response combinations help the daily activities of living beings, and specific biological structures guarantee high efficiency.
One often-neglected feature of signal transmission in neurons is the myelin sheath. This team found a reason why the sheath optimizes information transmission: it restricts ionic leakage through the membrane to specific points called Nodes of Ranvier. This gives the axon a “saltatory” conduction mechanism: “a unique action potential propagation pattern, which is limited in myelinated axons and demonstrates an ultrafast signal transmission ability.”
By reducing ionic spread in this way, the axon effectively makes the nervous system an AC (alternating current) system instead of DC (direct current) system. DC transmits by sine waves, but the neuron, using myelin sheaths, converts its ion channels into a kind of FM (frequency modulation), granting it much greater information-carrying capacity or bandwidth.
The researchers were inspired to imitate this electronically. Once again, nature had it first. And by using ions instead of electrons, biological signal conduction systems do not short out when the baby turtle dives into the water for the first time after hatching.
Random access through random bits would be a very inefficient way to find something. Imagine if every house had a random number instead of a proper street address; the post office would go crazy. A major reason the postman can find your house is that the address is hierarchically arranged: zip code (itself a five-digit hierarchy), state, city, street, house number, apartment number. Search engines provide rapid answers that way, too, using indexing methods. It turns out that we humans have such a system inside our skulls. The National Research University Higher School of Economics, reporting through Phys.org, finds that “Visual working memory is hierarchically structured,” even though the mass of neurons in the brain doesn’t outwardly appear that way.
In short-term memory, most people can only recall 3 or 4 objects that they have recently seen. Neurologists have debated whether individual sights are stored independently or in a hierarchical manner, where each object is embedded in an ensemble of objects to be recalled. Tests on participants asked to recall the orientations of triangles demonstrated the latter:
The researchers found out that the precision of an individual orientation report depended on the variability of all orientations. Furthermore, there was a significant resemblance between how accurately participants could remember the orientations of the items in the display and how clustered the orientations to be remembered were.
“This says for a fact that even as we try to memorize items individually, our working memory also stores the summary of the whole group,” commented Igor Utochkin, professor at the HSE School of Psychology. If precise information about a specific item isn’t in memory, one uses the ensemble statistics to recall the approximate characteristics of the object. The more precise these statistics are, the more precise the response concerning that object is.
The “ensemble statistics” thus provide an indexing algorithm to get to the item in memory more rapidly. The paper on these findings is published in the Journal of Experimental Psychology.6
As neuroscience advances, it will be interesting to see what algorithms are used for long-term memory storage and recall. Potentially everything one sees and experiences over a lifetime is stored in the brain. Much of it can be recalled quickly; some answers take longer. Everyone has had the experience of wondering, “What was the name of that neighbor when we were kids?” and getting the answer moments later, sometimes after turning attention to something else. Some type of search algorithm must be at work in the background that combs through a vast quantity of data that would challenge Google. Is the information stored hierarchically? Does the algorithm use indexing methods? Based on what the above research shows about biological hierarchies, it would not be surprising to learn that optimized algorithms are involved.
In Defiance of Evolution
Hierarchical design is ubiquitous in biological systems. These are merely a few recent examples noted by researchers. By its very nature, hierarchical design defies the stepwise progress assumed to work by natural selection. It requires foresight: the ability to envision a coordinated function in advance, and then the power to arrange materials and methods to achieve that function.
- Ning Xu et al., “A water lily–inspired hierarchical design for stable and efficient solar evaporation of high-salinity brine.” Science Advances 05 Jul 2019: Vol. 5, no. 7, eaaw7013. DOI: 10.1126/sciadv.aaw7013.
- Yaolei Xiang et al., “Superrepellency of underwater hierarchical structures on Salvinia leaf.” PNAS February 4, 2020 117 (5) 2282-2287; DOI: 10.1073/pnas.1900015117.
- Zhang et al., “Biologically inspired flexible photonic films for efficient passive radiative cooling.” PNAS June 30, 2020 117 (26) 14657-14666; June 15, 2020. DOI: 10.1073/pnas.2001802117.
- Laura Garcia-Perez et al., “Functional definition of a transcription factor hierarchy regulating T cell lineage commitment.” Science Advances 31 Jul 2020: Vol. 6, no. 31, eaaw7313. DOI: 10.1126/sciadv.aaw7313.
- Teng et al., “Bioinspired nervous signal transmission system based on two-dimensional laminar nanofluidics: From electronics to ionics.” PNAS July 21, 2020 117 (29) 16743-16748; July 1, 2020. DOI:10.1073/pnas.2005937117.
- Utochkin and Brady, “Individual representations in visual working memory inherit ensemble properties.” Journal of Experimental Psychology: Human Perception and Performance, 46(5), 458–473. DOI: 10.1037/xhp0000727.