Eukaryote

Etymology

The word eukaryote is derived from the Greek words "eu" (εὖ) meaning "true" or "good" and "karyon" (κάρυον) meaning "nut" or "kernel", referring to the nucleus of a cell.

Diversity

Eukaryotes are organisms that range from microscopic single cells, such as picozoans under 3 micrometres across, to animals like the blue whale, weighing up to 190 tonnes and measuring up to 33.6 m long, or plants like the coast redwood, up to 120 m tall. Many eukaryotes are unicellular; the informal grouping called protists includes many of these, with some multicellular forms like the giant kelp up to 200 ft long. The multicellular eukaryotes include the animals, plants, and fungi, but again, these groups too contain many unicellular species. Eukaryotic cells are typically much larger than those of prokaryotes—the bacteria and the archaea—having a volume of around 10,000 times greater. Eukaryotes represent a small minority of the number of organisms, but, as many of them are much larger, their collective global biomass (468 gigatons) is far larger than that of prokaryotes (77 gigatons), with plants alone accounting for over 81% of the total biomass of Earth.

The eukaryotes are a diverse lineage, consisting mainly of microscopic organisms. Multicellularity in some form has evolved independently at least 25 times within the eukaryotes. Complex multicellular organisms, not counting the aggregation of amoebae to form slime molds, have evolved within only six eukaryotic lineages: animals, symbiomycotan fungi, brown algae, red algae, green algae, and land plants. Eukaryotes are grouped by genomic similarities, so that groups often lack visible shared characteristics.

File:Gram-negative Bacteria and Paramecium forming cyst.jpg|Prokaryotes (small cylindrical cells, bacteria, on left) and a single-celled eukaryote, Paramecium File:California Redwood National Park (216450575).jpeg|Coast redwood File:Anim1754 - Flickr - NOAA Photo Library (rotated).jpg|Blue whale

Distinguishing features

Nucleus

The defining feature of eukaryotes is that their cells have a well-defined, membrane-bound nucleus, distinguishing them from prokaryotes that lack such a structure. Eukaryotic cells have a variety of internal membrane-bound structures, called organelles, and a cytoskeleton which defines the cell's organization and shape. The nucleus stores the cell's DNA, which is divided into linear bundles called chromosomes; these are separated into two matching sets by a microtubular spindle during nuclear division, in the distinctively eukaryotic process of mitosis.

Biochemistry

Eukaryotes differ from prokaryotes in multiple ways, with unique biochemical pathways such as sterane synthesis. The eukaryotic signature proteins have no homology to proteins in other domains of life, but appear to be universal among eukaryotes. They include the proteins of the cytoskeleton, the complex transcription machinery, the membrane-sorting systems, the nuclear pore, and some enzymes in the biochemical pathways.

Internal membranes

Eukaryote cells include a variety of membrane-bound structures, together forming the endomembrane system. Simple compartments, called vesicles and vacuoles, can form by budding off other membranes. Many cells ingest food and other materials through a process of endocytosis, where the outer membrane invaginates and then pinches off to form a vesicle. Some cell products can leave in a vesicle through exocytosis.

The nucleus is surrounded by a double membrane known as the nuclear envelope, with nuclear pores that allow material to move in and out. Various tube- and sheet-like extensions of the nuclear membrane form the endoplasmic reticulum, which is involved in protein transport and maturation. It includes the rough endoplasmic reticulum, covered in ribosomes which synthesize proteins; these enter the interior space or lumen. Subsequently, they generally enter vesicles, which bud off from the smooth endoplasmic reticulum. In most eukaryotes, these protein-carrying vesicles are released and their contents further modified in stacks of flattened vesicles (cisternae), the Golgi apparatus.

Vesicles may be specialized; for instance, lysosomes contain digestive enzymes that break down biomolecules in the cytoplasm.

Mitochondria

Main article: Mitochondrion

Mitochondria are organelles in eukaryotic cells. The mitochondrion is commonly called "the powerhouse of the cell",. for its function providing energy by oxidising sugars or fats to produce the energy-storing molecule ATP. Mitochondria have two surrounding membranes, each a phospholipid bilayer, the inner of which is folded into invaginations called cristae where aerobic respiration takes place.

Mitochondria contain their own DNA, which has close structural similarities to bacterial DNA, from which it originated, and which encodes rRNA and tRNA genes that produce RNA which is closer in structure to bacterial RNA than to eukaryote RNA.

Some eukaryotes, such as the metamonads Giardia and Trichomonas, and the amoebozoan Pelomyxa, appear to lack mitochondria, but all contain mitochondrion-derived organelles, like hydrogenosomes or mitosomes, having lost their mitochondria secondarily. They obtain energy by enzymatic action in the cytoplasm. It is thought that mitochondria developed from prokaryotic cells which became endosymbionts living inside eukaryotes.

Plastids

Main article: Plastid

Plants and various groups of algae have plastids as well as mitochondria. Plastids, like mitochondria, have their own DNA and are developed from endosymbionts, in this case cyanobacteria. They usually take the form of chloroplasts which, like cyanobacteria, contain chlorophyll and produce organic compounds (such as glucose) through photosynthesis. Others are involved in storing food. Although plastids probably had a single origin, not all plastid-containing groups are closely related. Instead, some eukaryotes have obtained them from other eukaryotes through secondary endosymbiosis or ingestion. The capture and sequestering of photosynthetic cells and chloroplasts, kleptoplasty, occurs in many types of modern eukaryotic organisms.

Cytoskeletal structures

Main article: Cytoskeleton

The cytoskeleton provides stiffening structure and points of attachment for motor structures that enable the cell to move, change shape, or transport materials. The motor structures are microfilaments of actin and actin-binding proteins. These include α-actinin, fimbrin, and filamin in submembranous cortical layers and bundles. Motor proteins of microtubules, dynein and kinesin, and myosin of actin filaments, make the network dynamic.

Many eukaryotes have long slender motile cytoplasmic projections, called flagella, or multiple shorter structures called cilia. These organelles are variously involved in movement, feeding, and sensation. They are composed mainly of tubulin, and are entirely distinct from prokaryotic flagella. They are supported by a bundle of microtubules arising from a centriole, characteristically arranged as nine doublets surrounding two singlets. Flagella may have hairs (mastigonemes), as in many stramenopiles. Their interior is continuous with the cell's cytoplasm.

Centrioles are often present, even in cells and groups that do not have flagella, but conifers and flowering plants have neither. They generally occur in groups that give rise to various microtubular roots. These form a primary component of the cytoskeleton, and are often assembled over the course of several cell divisions, with one flagellum retained from the parent and the other derived from it. Centrioles produce the spindle during nuclear division.

Cell wall

Main article: Cell wall

The cells of plants, algae, fungi and most chromalveolates, but not animals, are surrounded by a cell wall. This is a layer outside the cell membrane, providing the cell with structural support, protection, and a filtering mechanism. The cell wall also prevents over-expansion when water enters the cell.

The major polysaccharides making up the primary cell wall of land plants are cellulose, hemicellulose, and pectin. The cellulose microfibrils are linked together with hemicellulose, embedded in a pectin matrix. The most common hemicellulose in the primary cell wall is xyloglucan.

Sexual reproduction

Eukaryotes have a life cycle that involves sexual reproduction, alternating between a haploid phase, where only one copy of each chromosome is present in each cell, and a diploid phase, with two copies of each chromosome in each cell. The diploid phase is formed by fusion of two haploid gametes, such as eggs and spermatozoa, to form a zygote; this may grow into a body, with its cells dividing by mitosis, and at some stage produce haploid gametes through meiosis, a division that reduces the number of chromosomes and creates genetic variability. There is considerable variation in this pattern. Plants have both haploid and diploid multicellular phases. Eukaryotes have lower metabolic rates and longer generation times than prokaryotes, because they are larger and therefore have a smaller surface area to volume ratio.

The evolution of sexual reproduction may be a primordial characteristic of eukaryotes. Based on a phylogenetic analysis, Dacks and Roger have proposed that facultative sex was present in the group's common ancestor. A core set of genes that function in meiosis is present in both Trichomonas vaginalis and Giardia intestinalis, two organisms previously thought to be asexual. Since these two species are descendants of lineages that diverged early from the eukaryotic evolutionary tree, core meiotic genes, and hence sex, were likely present in the common ancestor of eukaryotes. Species once thought to be asexual, such as Leishmania parasites, have a sexual cycle. Amoebae, previously regarded as asexual, may be anciently sexual; while present-day asexual groups could have arisen recently.

Evolution

History of classification

In antiquity, the two lineages of animals and plants were recognized by Aristotle and Theophrastus. The lineages were given the taxonomic rank of kingdom by Linnaeus in the 18th century. Though he included the fungi with plants with some reservations, it was later realized that they are quite distinct and warrant a separate kingdom. The various single-cell eukaryotes were originally placed with plants or animals when they became known. In 1818, the German biologist Georg A. Goldfuss coined the word Protozoa to refer to organisms such as ciliates, and this group was expanded until Ernst Haeckel made it a kingdom encompassing all single-celled eukaryotes, the Protista, in 1866. The eukaryotes thus came to be seen as four kingdoms:

• Kingdom Protista
• Kingdom Plantae
• Kingdom Fungi
• Kingdom Animalia

The protists were at that time thought to be "primitive forms", and thus an evolutionary grade, united by their primitive unicellular nature. Understanding of the oldest branchings in the tree of life only developed substantially with DNA sequencing, leading to a system of domains rather than kingdoms as top level rank being put forward by Carl Woese, Otto Kandler, and Mark Wheelis in 1990, uniting all the eukaryote kingdoms in the domain "Eucarya", stating, however, that eukaryotes' will continue to be an acceptable common synonym". In 1996, the evolutionary biologist Lynn Margulis proposed to replace kingdoms and domains with "inclusive" names to create a "symbiosis-based phylogeny", giving the description "Eukarya (symbiosis-derived nucleated organisms)".

Phylogeny

Phylogeny

By the early 21st century, a rough consensus started to emerge from phylogenomic studies. The majority of eukaryotes can be placed in one of two large clades dubbed Amorphea (similar in composition to the unikont hypothesis) and the Diphoda (formerly bikonts), which includes plants and most algal lineages. A third major grouping, the Excavata, has been abandoned as a formal group as it was found to be paraphyletic. The proposed phylogeny below includes two groups of excavates (Discoba and Metamonada), and incorporates the 2021 proposal that picozoans are close relatives of rhodophytes. The Provora are a group of microbial predators discovered in 2022. TSAR is a possible clade that would contain Telonemia and the SAR supergroup.

Origin of eukaryotes

Eukaryogenesis Main article: Eukaryogenesis

The origin of the eukaryotic cell, or eukaryogenesis, is a milestone in the evolution of life, since eukaryotes include all complex cells and almost all multicellular organisms. The last eukaryotic common ancestor (LECA) is the hypothetical origin of all living eukaryotes, and was most likely a biological population, not a single individual. The LECA is believed to have been a protist with a nucleus, at least one centriole and flagellum, facultatively aerobic mitochondria, sex (meiosis and syngamy), a dormant cyst with a cell wall of chitin or cellulose, and peroxisomes.

An endosymbiotic union between a motile anaerobic archaean and an aerobic alphaproteobacterium gave rise to the LECA and all eukaryotes with mitochondria. A second, much later endosymbiosis with a cyanobacterium gave rise to the ancestor of plants, with chloroplasts.

The presence of eukaryotic biomarkers in archaea points towards an archaeal origin, except for mitochondrial DNA, which is bacterial in origin. The genomes of Promethearchaeota archaea have plenty of eukaryotic signature protein genes, which play a crucial role in the development of the cytoskeleton and complex cellular structures characteristic of eukaryotes. In 2022, cryo-electron tomography demonstrated that Promethearchaeota archaea have a complex actin-based cytoskeleton, providing the first direct visual evidence of the archaeal ancestry of eukaryotes.

Fossils

The timing of the origin of eukaryotes is hard to determine, but that of the earliest multicellular eukaryote so far discovered, Qingshania magnifica of North China, which lived 1.635 billion years ago, suggests that the crown group eukaryotes originated in the late Paleoproterozoic (Statherian). The earliest unequivocal unicellular eukaryotes, Tappania plana, Shuiyousphaeridium macroreticulatum, Dictyosphaera macroreticulata, Germinosphaera alveolata, and Valeria lophostriata from North China, lived approximately 1.65 billion years ago.

Some acritarchs are known from at least 1.65 billion years ago, and a fossil, Grypania, which may be an alga, is as much as 2.1 billion years old. The "problematic" fossil Diskagma has been found in paleosols 2.2 billion years old.

The Neoarchean fossil Thuchomyces shares some similarities with fungi. It especially resembles the problematic fossil Diskagma, with hyphae and multiple differentiated layers. However, it is over 600 million years older than all other possible eukaryotes, and many of its "eukaryote features" are not specific to the clade, meaning it is almost certainly a microbial mat instead.

Structures proposed to represent "large colonial organisms" have been found in the black shales of the Palaeoproterozoic such as the Francevillian B Formation, in Gabon, dubbed the "Francevillian biota" which is dated at 2.1 billion years old. However, the status of these structures as fossils is contested, with other authors suggesting that they might represent pseudofossils. The oldest fossils that can unambiguously be assigned to eukaryotes are from the Ruyang Group of China, dating to approximately 1.8-1.6 billion years ago. Fossils that are clearly related to modern groups start appearing an estimated 1.2 billion years ago, in the form of red algae, though recent work suggests the existence of fossilized filamentous algae in the Vindhya basin dating back perhaps to 1.6 to 1.7 billion years ago.

The presence of steranes, eukaryotic-specific biomarkers, in Australian shales previously indicated that eukaryotes were present in these rocks dated at 2.7 billion years old, but these Archaean biomarkers have been rebutted as later contaminants. The oldest valid biomarker records are only around 800 million years old. In contrast, a molecular clock analysis suggests the emergence of sterol biosynthesis as early as 2.3 billion years ago. The nature of steranes as eukaryotic biomarkers is further complicated by the production of sterols by some bacteria.

Whenever their origins, eukaryotes may not have become ecologically dominant until much later; a massive increase in the zinc composition of marine sediments has been attributed to the rise of substantial populations of eukaryotes, which preferentially consume and incorporate zinc relative to prokaryotes, approximately a billion years after their origin (at the latest).

References

References

1. {{Cite Merriam-Webster. eukaryote
2. "eukaryotic (adj.)". [[Online Etymology Dictionary]].
3. (26 March 2013). "Picomonas judraskeda Gen. Et Sp. Nov.: The First Identified Member of the Picozoa Phylum Nov., a Widespread Group of Picoeukaryotes, Formerly Known as 'Picobiliphytes'". PLOS ONE.
4. Wood, Gerald. (1983). "The Guinness Book of Animal Facts and Feats". [[Guinness World Records]].
5. (2017). "Sequoia sempervirens". The Gymnosperm Database.
6. (1995). "Algae An Introduction to Phycology". [[Cambridge University Press]].
7. (May 2014). "The eukaryotic tree of life from a global phylogenomic perspective". Cold Spring Harbor Perspectives in Biology.
8. DeRennaux, B.. (2001). "Encyclopedia of Biodiversity". [[Elsevier]].
9. (2014). "Deep-sea microorganisms and the origin of the eukaryotic cell". Japanese Journal of Protozoology.
10. (2018-05-17). "The biomass distribution on Earth". [[Proceedings of the National Academy of Sciences of the United States of America.
11. (2020). "The New Tree of Eukaryotes". Trends in Ecology & Evolution.
12. (2007). "The evolution of multicellularity: A minor major transition?". [[Annu Rev Ecol Evol Syst]].
13. (2013). "Multicellularity arose several times in the evolution of eukaryotes". BioEssays.
14. (2011). "Evolution and diversity of plant cell walls: From algae to flowering plants". Annual Review of Plant Biology.
15. (14 October 2016). "Organization and function of the 3D genome". Nature Reviews Genetics.
16. O'Connor, Clare. (2008). "Chromosome Segregation: The Role of Centromeres". Nature Education.
17. (February 2002). "The origin of the eukaryotic cell: a genomic investigation". Proceedings of the National Academy of Sciences of the United States of America.
18. (2011). "Functional Genomics and Evolution of Photosynthetic Systems". Springer.
19. (2001). "Endocytosis". [[Oxford University Press]].
20. (November 2020). "Direct trafficking pathways from the Golgi apparatus to the plasma membrane". Seminars in Cell & Developmental Biology.
21. (March 2010). "The nuclear envelope". Cold Spring Harbor Perspectives in Biology.
22. "Endoplasmic Reticulum (Rough and Smooth)". British Society for Cell Biology.
23. "Golgi Apparatus". British Society for Cell Biology.
24. "Lysosome". British Society for Cell Biology.
25. (July 1957). "Powerhouse of the Cell". Scientific American.
26. (2006). "Fundamentals of Biochemistry". [[John Wiley and Sons]].
27. (1 May 2006). "Re: Are there eukaryotic cells without mitochondria?". madsci.org.
28. (January 2009). "Cristae formation-linking ultrastructure and function of mitochondria.". Biochimica et Biophysica Acta (BBA) - Molecular Cell Research.
29. (1988). "Molecular Biology of the Gene". [[Benjamin Cummings.
30. (13 May 2016). "Scientists Shocked To Discover Eukaryote With NO Mitochondria".
31. (May 2016). "A Eukaryote without a Mitochondrial Organelle". Current Biology.
32. (October 2021). "The Genomics and Cell Biology of Host-Beneficial Intracellular Infections". Annual Review of Cell and Developmental Biology.
33. (2006). "The Structure and Function of Plastids". [[Springer Netherlands]].
34. (September 2008). "Chloroplast DNA content in Dinophysis (Dinophyceae) from different cell cycle stages is consistent with kleptoplasty". Environ. Microbiol..
35. (February 2018). "Did some red alga-derived plastids evolve via kleptoplastidy? A hypothesis". [[Biological Reviews of the Cambridge Philosophical Society]].
36. (2002-01-01). "Molecular Biology of the Cell". [[Garland Science]].
37. (May 2018). "Motor Proteins". Cold Spring Harbor Perspectives in Biology.
38. (February 2003). "Prokaryotic motility structures". Microbiology.
39. (December 2001). "Assembly and motility of eukaryotic cilia and flagella. Lessons from Chlamydomonas reinhardtii". Plant Physiology.
40. (1987). "The Centrosome and Its Role in the Organization of Microtubules".
41. (2000). "The Surprising Archaea: Discovering Another Domain of Life". [[Oxford University Press]].
42. (1989). "The Structure and Functions of Xyloglucan". Journal of Experimental Botany.
43. Hamilton, Matthew B.. (2009). "Population genetics". [[Wiley-Blackwell]].
44. (2005). "Life history biology of early land plants: Deciphering the gametophyte phase". Proceedings of the National Academy of Sciences of the United States of America.
45. (June 2011). "Energetics and genetics across the prokaryote-eukaryote divide". Biology Direct.
46. (June 1999). "The first sexual lineage and the relevance of facultative sex". Journal of Molecular Evolution.
47. (January 2005). "A phylogenomic inventory of meiotic genes; evidence for sex in Giardia and an early eukaryotic origin of meiosis". Current Biology.
48. (August 2007). "An expanded inventory of conserved meiotic genes provides evidence for sex in Trichomonas vaginalis". [[PLOS One]].
49. (April 2009). "Demonstration of genetic exchange during cyclical development of Leishmania in the sand fly vector". Science.
50. (July 2011). "The chastity of amoebae: re-evaluating evidence for sex in amoeboid organisms". [[Proceedings: Biological Sciences]].
51. (1980). "Taxonomic proposals for the classification of marine yeasts and other yeast-like fungi including the smuts". Botanica Marina.
52. Goldfuß. (1818). "Ueber die Classification der Zoophyten". Isis, Oder, Encyclopädische Zeitung von Oken.
53. (1999). "Not plants or animals: a brief history of the origin of Kingdoms Protozoa, Protista and Protoctista". [[International Microbiology]].
54. (1989). "Protozoa, Protista, Protoctista: what's in a name?". Journal of the History of Biology.
55. (January 1969). "New concepts of kingdoms or organisms. Evolutionary relations are better represented by new classifications than by the traditional two kingdoms". Science.
56. (June 1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya". Proceedings of the National Academy of Sciences of the United States of America.
57. Knoll, Andrew H.. (1992). "The Early Evolution of Eukaryotes: A Geological Perspective". Science.
58. Margulis, Lynn. (6 February 1996). "Archaeal-eubacterial mergers in the origin of Eukarya: phylogenetic classification of life". Proceedings of the National Academy of Sciences.
59. (January 2016). "Untangling the early diversification of eukaryotes: a phylogenomic study of the evolutionary origins of Centrohelida, Haptophyta and Cryptista". Proceedings: Biological Sciences.
60. (21 November 2023). "Openly available illustrations as tools to describe eukaryotic microbial diversity". PLOS Biology.
61. (January 2019). "Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes". The Journal of Eukaryotic Microbiology.
62. (2018-01-19). "Phylogenomics Places Orphan Protistan Lineages in a Novel Eukaryotic Super-Group". Genome Biology and Evolution.
63. (2021). "Picozoa are archaeplastids without plastid". Nature Communications.
64. (December 2022). "Microbial predators form a new supergroup of eukaryotes". Nature.
65. (April 2019). "New Phylogenomic Analysis of the Enigmatic Phylum Telonemia Further Resolves the Eukaryote Tree of Life". Molecular Biology and Evolution.
66. (2022). "The closest lineage of Archaeplastida is revealed by phylogenomics analyses that include Microheliella maris". Open Biology.
67. (27 August 2024). "Phylogenomics of neglected flagellated protists supports a revised eukaryotic tree of life". Current Biology.
68. (October 2021). "Origin and Early Evolution of the Eukaryotic Cell". Annual Review of Microbiology.
69. (May 2020). "Predatory protists". Current Biology.
70. (March 2021). "A molecular timescale for eukaryote evolution with implications for the origin of red algal-derived plastids". Nature Communications.
71. (2013). "Molecular paleontology and complexity in the last eukaryotic common ancestor". Critical Reviews in Biochemistry and Molecular Biology.
72. (2011). "Origins and Evolution of Life: An astrobiological perspective". [[Cambridge University Press]].
73. (2023). "Actin cytoskeleton and complex cell architecture in an Asgard archaean". Nature.
74. (2024). "1.63-billion-year-old multicellular eukaryotes from the Chuanlinggou Formation in North China". Science Advances.
75. (July 1992). "Megascopic eukaryotic algae from the 2.1-billion-year-old negaunee iron-formation, Michigan". Science.
76. (June 2006). "Eukaryotic organisms in Proterozoic oceans". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences.
77. (2013). "Problematic urn-shaped fossils from a Paleoproterozoic (2.2 Ga) paleosol in South Africa.". Precambrian Research.
78. (December 1977). "Morphological and anatomical observations on some precambrian plants from the Witwatersrand, South Africa". Geologische Rundschau.
79. (2018). "Ediacarans, Protolichens, and Lichen-Derived Penicillium". Transformative Paleobotany.
80. (July 2010). "Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago". Nature.
81. El Albani, Abderrazak. (2023). "A search for life in Palaeoproterozoic marine sediments using Zn isotopes and geochemistry". Earth and Planetary Science Letters.
82. (2023). "Zinc enrichment and isotopic fractionation in a marine habitat of the c. 2.1 Ga Francevillian Group: A signature of zinc utilization by eukaryotes?". Earth and Planetary Science Letters.
83. (May 2023). "Earth's surface oxygenation and the rise of eukaryotic life: Relationships to the Lomagundi positive carbon isotope excursion revisited". Earth-Science Reviews.
84. (May 2009). "The controversial "Cambrian" fossils of the Vindhyan are real but more than a billion years older". Proceedings of the National Academy of Sciences of the United States of America.
85. (August 1999). "Archean molecular fossils and the early rise of eukaryotes". Science.
86. (9 February 2008). "Mass extinctions: the microbes strike back".
87. (May 2015). "Reappraisal of hydrocarbon biomarkers in Archean rocks". Proceedings of the National Academy of Sciences of the United States of America.
88. (August 2017). "The rise of algae in Cryogenian oceans and the emergence of animals". Nature.
89. (March 2017). "Paleoproterozoic sterol biosynthesis and the rise of oxygen". Nature.
90. (2016-06-24). "Sterol Synthesis in Diverse Bacteria". Frontiers in Microbiology.
91. (June 2021). "Evolution of bacterial steroid biosynthesis and its impact on eukaryogenesis". Proceedings of the National Academy of Sciences of the United States of America.
92. (June 2018). "Tracking the rise of eukaryotes to ecological dominance with zinc isotopes". Geobiology.