Friday 30 June 2023
Still no simple beginning
On the Irreducible Complexity of Sperm Cells
Human reproduction is perhaps the quintessential example of teleology in biology. The process by which a fertilized egg develops into an infant over the space of nine months reveals exquisite engineering and ingenious design. Before this intricate process can even begin, there is a need for a sperm cell to fuse with an ovum — each carrying, in the case of humans, 23 chromosomes. This incredible feat bears the unmistakable hallmarks of conscious intent and foresight
Here , I will focus on the design characteristics of sperm cells. In a subsequent post, I will discuss the design features of the seminal fluid, and sperm capacitation. Sperm cells are comprised of three components — the head, the middle piece, and the flagellum — and hundreds of millions of them are carried in the seminal fluid that is released into the cervix through ejaculation during sexual intercourse. With each ejaculation, the male releases between two hundred and five hundred million sperm cells (approximately 100 million per milliliter of semen). Each of these three components, and the seminal fluid, is crucial to the sperm cell’s mission of fusing with an ovum to form a zygote (a fertilized egg). Let us consider each one in turn.
The Head
The head carries densely coiled chromatin fibers, containing the haploid genome — totaling half of the genetic material that will be inherited by the next generation (the other half will come from the mother’s egg cell). The tight packaging of the DNA serves to minimize its volume for transport.
On the tip of the sperm head is a membranous organelle, called the acrosome, that contains various hydrolytic enzymes. When these are secreted, they digest the egg cell membrane, thereby facilitating penetration of the ovum. Without the acrosome, the sperm cell will be unable to penetrate the egg cell membrane to fertilize the ovum. According to a review paper published in Frontiers in Cell and Developmental Biology:
Any structural or functional acrosomal abnormality could impair sperm fusion, and ultimately result in infertility. Moreover, studies have shown that intra-cytoplasmic insemination with sperm containing acrosomal abnormalities did not lead to successful fertilization, even in the absence of fertilization barriers, because the oocyte was unable to be efficiently activated…Thus, the acrosome is indispensable for fertilization.1
When a sperm reaches the vicinity of the egg, it undergoes a series of molecular interactions with the zona pellucida, which is a specialized extracellular matrix surrounding the egg. Specific receptors on the sperm’s plasma membrane, such as spermadhesins or integrins, recognize and bind to corresponding ligands on the zona pellucida. This binding triggers the activation of signaling pathways in the sperm. Binding of the sperm receptors to the zona pellucida ligands leads to an influx of calcium ions (Ca2+) into the sperm cell. This calcium influx is typically mediated by ion channels or receptors on the sperm’s plasma membrane, which are activated upon ligand-receptor binding. The increase in intracellular calcium levels initiates a signaling cascade within the sperm cell. Calcium ions act as second messengers and trigger the activation of various downstream signaling molecules and enzymes, including protein kinases. As a result of the calcium-mediated signaling cascade, the acrosome undergoes exocytosis. The membrane surrounding the acrosome fuses with the sperm’s plasma membrane, causing the release of the acrosomal contents, including enzymes such as hyaluronidase and acrosin. The enzymes released from the acrosome help degrade the glycoprotein matrix of the zona pellucida, allowing the sperm to penetrate and reach the egg’s plasma membrane. The acrosomal contents aid in the breakdown of the protective layers surrounding the egg, facilitating the fusion of the sperm and egg membranes.
The formation of the acrosome itself is divided into four stages. The first stage, the “Golgi phase,” is dependent upon the Golgi apparatus, which produces and packages the proteins and enzymes needed for acrosome formation. These proteins are then transported into the developing acrosome vesicle. In the second phase, the “cap phase,” the Golgi-derived vesicle (known as the proacrosomal vesicle) fuses with the anterior portion of the nucleus, forming a cap-like structure over the nucleus. The fusion of the vesicle with the nucleus is mediated by membrane trafficking processes. The proacrosomal vesicle contains enzymes, glycoproteins, and other components that are essential for acrosome maturation. In the third phase, the “acrosome phase,” the cap-like structure undergoes a series of structural changes, leading to the formation of the acrosome. The proacrosomal vesicle flattens and elongates, spreading over the anterior region of the nucleus. The Golgi-derived enzymes modify the proteins present in the proacrosomal vesicle, converting them into their active forms. The acrosomal membrane also undergoes changes, becoming specialized for the acrosome’s functions. In the final phase, the “maturation phase,” the acrosome undergoes further modifications and maturation. Enzymes within the acrosome become fully activated and the acrosomal matrix undergoes changes, becoming more condensed. The acrosomal granule, which is the central region of the acrosome, becomes highly electron-dense due to the accumulation of enzymes and proteins. The mature acrosome is now ready for its role in fertilization. For a more detailed description of this incredible process, I refer readers to a review paper on the “Mechanism of Acrosome Biogenesis in Mammals.”2
The Middle Piece
The middle piece consists of a central filamentous core, around which are many strategically placed mitochondria that synthesize the energy molecule adenosine triphosphate (ATP). The complexity and design of energy generation within the mitochondria — including the processes of glycolysis, the citric acid (or, Krebs) cycle, the electron transport chain, and oxidative phosphorylation — could be its own series of articles, but this is a topic for another day. For a good introduction to the phenomenal processes within the mitochondria, here are three animations from Harvard University that bring this fascinating organelle to life:
“Mitochondria: The Cell’s Powerhouse”
“Electron Transport Chain”
“ATP Synthase in action”
The ATP generated by the mitochondria energizes the power strokes of the flagellum, driving its journey through the female cervix, uterus, and uterine tubes. As such, the middle piece of the sperm cell is absolutely essential to its function of swimming through the female uterus and fallopian tube to fertilize her egg. Without the middle piece and its mitochondria, the sperm cells are completely immobile.
The Flagellum
Unlike a bacterial flagellum (which rotates like a motor), a sperm flagellum beats with a whip-like motion to produce motility. How does the flagellum work? In 2018, Jianfeng Lin and Daniela Nicastro elucidated the mechanism of flagellar motility.3 Their data indicated that “bending was generated by the asymmetric distribution of dynein activity on opposite sides of the flagellum”4 (dyneins are ATP-powered molecular motors that “walk” along microtubules towards their minus end). Their results also revealed that alternating flagellar bending occurs due to “a ‘switch-inhibition’ mechanism in which force imbalance is generated by inhibiting…dyneins on alternating sides of the flagellum.”5 In other words, regulatory signals lead to the inhibition of dynein motors on one side of the flagellum. Meanwhile, on the other side, the dyneins walk along the microtubules. The flagellum bends in one direction due to molecular linkers that resist this sliding. The flagellar bending alternates by repeatedly switching the side of dynein inhibition. Look here for an animation showing how this is thought to work.
It goes without saying that, without the flagellum, the sperm cell is completely immotile and has no chance of fertilizing the egg.
Thus far, we have considered the irreducible complexity of the components of a sperm cell. In a subsequent article, we shall consider the design features of the seminal fluid and the process of sperm capacitation that takes place within the female reproductive tract.
Notes
Khawar MB, Gao H, Li W. Mechanism of Acrosome Biogenesis in Mammals. Front Cell Dev Biol. 2019 Sep 18;7:195.
Ibid.
Lin J, Nicastro D. Asymmetric distribution and spatial switching of dynein activity generates ciliary motility. Science. 2018 Apr 27;360(6387):eaar1968.
Ibid.
Ibid.
Continuing to massage the record?
Fossil Friday: Homo rudolfensis, Another Contentious Homo
Last week for Fossil Friday I posted some musings about Homo habilis and its controversial attribution to our genus. This week we will have a look at another disputed relative, Homo rudolfensis. Alexeev (1986) described the new species Homo (Pithecanthropus) rudolfensis from a single skull (KNM-ER 1470) discovered in 1972 by Richard Leakey at the 1.9 million year old Koobi Fora locality of the Turkana Lake (formerly known as Lake Rudolf) basin in East Africa (also see Wood 1999). The material was previously considered to be conspecific with Homo habilis, which is a hypothesis still entertained by some modern experts. However, the skull differs from Homo habilis in its flat face and larger brain volume as well as the more robust-australopithecine-like cheek teeth. Unfortunately, no associated postcranial remains are known yet (Berger et al. 2015), so that the most distinctive characters of the genus Homo and those for bipedal gait are unknown (Tuttle 2006: 253).
A New Digital Reconstruction
As for Homo habilis, Wood & Collard (1999a, 1999b, 2001) and Collard & Wood (2007, 2015) indeed advocated for transferring H. rudolfensis to the genus Australopithecus, which had already been suggested by other researchers (i.e., Walker 1976 and Lieberman et al. 1996). Walker & Shipman (1996) pointed out that “1470 might have a big braincase, but morphologically it was just an australopithecine.” A new digital reconstruction of the skull by Bromage et al. (2008) showed that it was somewhat less flat and the brain volume somewhat smaller, which made it even more similar to australopithecine skulls. Nevertheless, the latter study retained this species in the genus Homo. A co-author of this study was German paleoanthropologist Friedemann Schrenk, who at my university, Tübingen, was known by the sneering nickname the “Möllemann of German paleontology.” That was because he shared a notorious proclivity for PR stunts and media hype with the late German politician Jürgen Möllemann. He discovered a hominin mandible (UR 501) in Malawi, which he attributed to Homo rudolfensis and with an estimated age of 2.4 million years this would be much older than the holotype. Of course, publications on an early Homo make for much more sensational press releases than just another ape-man.
Anyway, Leakey et al. (2001) and Lieberman (2001) noted several striking similarities in the facial architecture of the newly described hominin Kenyanthropus platyops and the 1.6 million year younger H. rudolfensis, who could be a late survivor of the australopithecine-like Kenyanthropus lineage rather than an early Homo. The phylogenetic analysis by Cameron & Groves (2004) strongly confirmed the reclassification as Kenyanthropus rudolfensis by Cameron (2003). Cela-Conde & Ayala (2003) agreed that Homo rudolfensis (and H. habilis) should be grouped with Kenyanthropus platyops, but instead proposed to include all three within the genus Homo. That would place the origin of our genus 3.5 million years ago, in stark contradiction to all other experts and the unequivocal empirical evidence from the fossil record.
Four Hypotheses
Prat (2007) compared the four suggested alternative hypotheses: H. rudolfensis is conspecific with Homo habilis; H. rudolfensis and H. habilis are both distinct species of Homo; both species belong to the genus Australopithecus; or H. rudolfensis belongs to the genus Kenyanthropus. Prat came to the conclusion that Homo rudolfensis is distinct but her cladistic analysis suffers from several flaws. This is evident from the fact that the inclusion of the holotype of Kenyanthropus platyops did not just influence the polarity of some characters but produced a totally different tree topology with hardly any similarity to the tree recovered by excluding this taxon. The confidence level in any such highly unstable analyses should be very low for reasonable and unbiased scientists. However, having two early species of Homo is of course a highly desirable result for evolutionist paleoanthropologists, and so it is hardly surprising that almost all subsequent publications maintained the attribution of these two species to the genus Homo.
Awaiting Better Evidence
More recently, a more ancient origin of our genus has indeed been claimed by the discovery of a 2.8 million year old human mandible at Ledi-Geraru in the Afar region of Ethiopia, which was attributed to an early Homo (Villmoare et al. 2015). But this fossil combines primitive australopithecine traits with more derived features of later Homo, and it also suffers from the absence of any other cranial and postcranial characters that could support this claim. Considering the checkered history of grandiose claims and controversies in paleoanthropology, some caution may be wise until more and better evidence is found.
References
Alexeev VP 1986. The Origin of the Human Race. Progress Publishers, Moscow, 360 pp. https://archive.org/details/originhumanrace/page/1/mode/2up
Berger LR, Hawks J, de Ruiter DJ et al. 2015. Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa. eLife 4:e09560, 1–35. DOI: https://doi.org/10.7554/eLife.09560
Bromage TG, McMahon JM, Thackeray JF, Kullmer O, Hogg R, Rosenberger AL, Schrenk F & Enlow DH 2008. Craniofacial architectural constraints and their importance for reconstructing the early Homo skull KNM-ER 1470. Journal of Clinical Pediatric Dentistry 33, 43–54. DOI: https://doi.org/10.17796/jcpd.33.1.8168115j12103nut
Cameron DW 2003. Early hominin speciation at the Plio/Pleistocene transition. HOMO 54(1), 1–28. DOI: https://doi.org/10.1078/0018-442X-00057
Cameron DW & Groves CP 2004. Bones, Stones, and Molecules: “Out of Africa” and Human Origins. Academic Press, Burlington (MA), xi+402 pp.
Cela-Conde CJ & Ayala FJ 2003. Genera of the human lineage. PNAS 100(13), 7684–7689.
DOI: https://doi.org/10.1073/pnas.0832372100
Collard M & Wood B 2007. Defining the Genus Homo. pp. 1575–1610 in: Henke W & Tattersall I (eds). Handbook of Paleoanthropology. 3 vols. Springer, Berlin, 2069 pp.
Collard M & Wood B 2015. Defining the Genus Homo. pp. 2107–2144 in: Henke W & Tattersall I (eds). Handbook of Paleoanthropology. 3 vols. Springer, Berlin, xliii+2624 pp. DOI: https://doi.org/10.1007/978-3-642-39979-4_51
Tuttle RH 2006. Are Human Beings Apes, or are Apes People too? pp. 249–258 in: Ishida H, Tuttle R, Pickford M, Ogihara N & Nakatsukasa M (eds). Human Origins and Environmental Backgrounds. Springer Science, Boston (MA), x+282 pp. DOI: https://doi.org/10.1007/0-387-29798-7_19
Leakey MG, Spoor F, Brown FH, Gathogo PN, Kiarie C, Leakey LN & McDougall I 2001. New hominin genus from eastern Africa shows diverse middle Pliocene lineages. Nature 410(6827), 433–440. DOI: https://doi.org/10.1038/35068500
Lieberman DE 2001. Another face in our family tree. Nature 410(6827), 419–420. DOI: https://doi.org/10.1038/35068648
Lieberman DE, Wood BA & Pilbeam DR 1996. Homoplasy and early Homo: An analysis of the evolutionary relationships of H. habilis sensu stricto and H. rudolfensis. Journal of Human Evolution 30, 97–120. DOI: https://doi.org/10.1006/jhev.1996.0008
Prat S 2007. The Quaternary boundary: 1.8 or 2.6 millions years old? Contributions of early Homo. Quaternaire 18(1), 99–107.
DOI: https://doi.org/10.4000/quaternaire.1313
Villmoare B, Kimbel WH, Seyoum C et al. 2015. Early Homo at 2.8 Ma from Ledi-Geraru, Afar, Ethiopia. Science 347(6228), 1352–1355. DOI: https://doi.org/10.1126/science.aaa1343
Walker A 1976. Remains attributable to Australopithecus in the East Rudolf succession. pp 484–489 in: Coppens Y, Howell FC, Isaac GL & Leakey REF (eds). Earliest Man and Environments in the Lake Rudolf Basin. University of Chicago Press, Chicago (IL), 640 pp.
Walker A & Shipman P 1996. The Wisdom of the Bones: In Search of Human Origins. Knopf, New York (NY), 368 pp.
Wood B 1999. Homo rudolfensis Alexeev, 1986: Fact or phantom?. Journal of Human Evolution 36(1), 115–118. DOI: https://doi.org/10.1006/jhev.1998.0246
Wood B & Collard M 1999a. The Human Genus. Science 284(5411), 65–71. DOI: https://doi.org/10.1126/science.284.5411.65
Wood B & Collard M 1999b. The changing face of genus Homo. Evolutionary Anthropology 8(6), 195–207. DOI: https://doi.org/10.1002/(SICI)1520-6505(1999)8:6<195::AID-EVAN1>3.0.CO;2-2
Wood B & Collard M 2001. The meaning of Homo. Ludus Vitalis 9(15), 63–74. http://profmarkcollard.com/wp-content/uploads/2014/09/Wood-and-Collard-2001.pdf
On the anti-Darwinian bias of the natural law.
Intelligence Is Unnatural, and Why That Matters
One of the advantages we have in our study of nature is our ability to observe an entire “unpolluted” universe. By “unpolluted” I mean that as we look out from Earth, we observe an almost unlimited theater of the natural. And what do we observe? Mostly empty space, visibly interspersed with galaxies composed of stars and nebulae. The regularities of the laws of nature also reveal unseen actors such as dark matter and energy, planets, and even black holes.
Speaking of the laws of nature, the heavenly stage extends so far away that light’s finite speed shows us scenes that happened in the past — from about one and one fourth seconds in the past, when we look at the moon, to more than 13 billion years ago in recent images of distant galaxies revealed by the James Webb Space Telescope. The physical universe provides astronomers with a time machine for viewing nature throughout the history of the cosmos. And what we see confirms the unchanging nature of the laws of physics.
What Spectroscopy Reveals
Using spectroscopy, astronomers not only observe the large-scale features of the universe, but through analysis of the specific wavelengths of electromagnetic radiation received, details of the atomic components of stars and gas clouds also are revealed. In the past as well as in the present, both near to home in our solar system and out to the most distant reaches of the visible universe, the same atomic characters fill the arena of the universe. Throughout the entire long history of the universe and in every direction we look, nature has only managed to produce a very limited playlist of elements — 92 different elements, from hydrogen with one proton as its nucleus to uranium with 92 protons.1
The reason I emphasize the limited number of types of elements in the entire universe is to suggest that one of the characteristics of nature is its “sameness” or redundancy. Now, there’s a reason for this: natural outcomes are governed by natural law. Only four fundamental forces of nature exist. Gravity pulls masses together; the electromagnetic force has twice the fun and can both push and pull masses that possess electric charge. The strong nuclear force also pulls,2 but with restrictions. It only acts on nucleons (protons and neutrons, but not electrons), and it has an extremely short range of about one fermi (10-15 m). The weak force neither pushes nor pulls, but is responsible for certain decay processes of elementary particles.
An example of the sameness of the cosmos is seen in the limited range of star masses. From at least a billion trillion stars in the universe, we find that their masses vary only over a range of about 1800, from 8 percent of the sun’s mass up to about 150 times the sun’s mass. These mass limits are not accidental; they are fixed by the laws of physics.
Seemingly Endless Variations
But what about the seemingly endless variations in the palette of sunset colors and patterns in the western sky? Doesn’t that run counter to the concept of limited diversity of natural phenomena? Certainly, we all appreciate the beauty of the rosy colorations of clouds illuminated by the rays of the setting sun. However, if we spent a thousand evenings watching the sun set, the sky would simply depict variations on a theme, with amorphous shapes of clouds shaded with gradations of color. Air, clouds, and light all respond according to the laws of nature, limiting their arrangements to forms devoid of specific complexity. Sameness prevails.
Turning our gaze away from sky and stars to the biosphere of Earth, we are struck by diversity unlimited. First, consider the unbelievable range of sizes and forms and behaviors of the millions of species of creatures that have lived on Earth. From tiny diatoms to enormous dinosaurs, from worms to eagles, and ants to people, the variety of living forms on Earth is astonishing compared to the overall sameness of the entire non-living universe.
The stark contrast between our life-filled planet and the rest of the cosmos sharpens further when we take into account all of the things produced by humans throughout our relatively short history on the stage of existence. Clocks, cars, computers, castles, clothing, and can openers. Besides physical creations, humans have produced a fantastic variety of musical and literary forms and coding for computer programs. Our prolific creativity seems limitless, and the scope of what we make spans an enormously broad spectrum with a variety that’s anything but “more of the same.”
The contrast between variety in living forms on just one planet, compared to the vast sameness of the non-living universe suggests a clear-cut distinction between even the simplest living organisms and non-living arrangements of matter. An objective consideration of the flourishing creativity of human endeavors compared to the routine instinctual behaviors of other creatures further suggests a categorical difference between human beings and other creatures.
A Physics Point of View
The exceptionalism manifested by humans, when compared to the predictably limited outcomes of non-living matter, is evidence that the choices and actions of intelligent beings are not governed by the laws of physics. My body is affected by gravity, but the force of gravity doesn’t determine what I eat for lunch. My cellular biochemistry is affected by the electromagnetic force, but my decision about what topic to address in my next article is not. The strong force holds the nuclei of my carbon atoms together, but it in no way determines what color my wife will choose to paint the living room.
How did we become so unnaturally creative? The Judeo-Christian tradition offers one possible answer. The belief that human beings are made in the image of God resonates with the unique creativity expressed by humanity throughout history. The more closely related the created is to the Creator, the more attributes of the one are to be expected in the other.
With our intelligent and creative minds, we can bring together the raw materials of the natural universe into an unlimited variety of forms that are both functional and purposive. Human expression manifests the unnatural attributes of creating art, literature, and technology — outcomes that would never arise by the influence of natural processes alone. Freedom and creativity complement one another; neither will flourish under controlling forces. If the forces of nature governed our thoughts and actions, would we see the vast panoply of creative human expression displayed throughout the history of civilization? It seems not.
Notes
A few more (or fewer) natural elements could be considered, depending on whether one includes those that are extremely rare or have a very short half-life.
The strong force becomes repulsive for inter-nucleon distances less than about 0.5 fermi.
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