In the beginning, there was boredom. Following the emergence of cellular life on earth, some 3.5 billion years ago, simple cells lacking a nucleus and other detailed internal structure dominated the planet. Matters would remain largely unchanged in terms of evolutionary development in these so-called prokaryotic cells — the bacteria and archaea — for another billion and a half years.
Then, something remarkable and unprecedented took place. A new type of cell, known as a eukaryote, emerged. The eukaryotes would evolve many complex internal modules or organelles, including the endoplasmic reticulum, the Golgi apparatus and the mitochondria, forming wildly diverse cell types — precursors to all subsequent plant and animal life on earth. Prokaryotic cells, which include bacteria and archaea, are structurally simple organisms, lacking the complex internal structure found in eukaryotes. All living plant and animal species today have their origins in the Last Eukaryotic Common Ancestor or LECA. The transition from prokaryote to eukaryote has remained a central mystery biologists are still trying to untangle.
How this crucial transition came to be remains a central mystery in biology.
The researchers explore in detail, the energy requirements of eukaryotic cells, which are on average, larger and more complex compared with prokaryotes. Their quantitative results stand in opposition to a reigning dogma, first put forward by biologists Nick Lane and Bill Martin.
Genesis to Revelation
The basic idea of Lane and Martin is that a cell’s developmental fate is governed by its supply of energy. Simple prokaryotes are mostly small and consist of single cells or small colonies and can subsist on more limited stores of energy to power their activities. But once a cell achieves sufficient size and complexity, it eventually reaches a barrier, beyond which such prokaryotes can not pass. Or so the theory has it.
According to this idea, a singular event in Earth’s history gave sudden rise to the eukaryotes, which then grew and diversified to occupy every ecological niche on the planet, from undersea vents to arctic tundra. This vast diversification occurred when a free-living prokaryotic cell acquired another tiny organism within the confines of its interior.
Through a process known as endosymbiosis, the new cell resident is taken up by this proto-eukaryote, supplying it with additional energy and enabling its transformation. The endosymbiont it has acquired would eventually develop into mitochondria — cellular powerhouses found only in eukaryotic cells.
Because all complex life today can be traced to a single eukaryotic branch of the evolutionary tree, it has been assumed that this chance endosymbiotic event, the acquisition of mitochondria, occurred once and only once during the entire history of life on Earth. This accident of nature is why we’re all here. Without mitochondria, the larger volume and complexity of eukaryotes would not be energetically viable.
Not so fast, the authors of the new study claim.
Crossing the borderlands
Schavemaker notes that while the distinction between prokaryotes and eukaryotes among organisms living today is obvious, things were murkier during the transition phase. Eventually, all the common traits of extant eukaryotes would be acquired, yielding an organism researchers refer to as LECA or the Last Eukaryotic Common Ancestor.
The new study explores the advent of the first eukaryotes and notes that instead of a hard boundary line separating them from their prokaryotic ancestors, the true picture is messier. Rather than an unbridgeable gulf between prokaryotes and eukaryotes in terms of cell volume internal complexity and number of genes, the two cell forms enjoyed considerable overlap.
The researchers investigate a range of prokaryotic and eukaryotic cell types to determine a) how cell volume in prokaryotes can eventually act to constrain a cell’s membrane surface area required for respiration, b) how much energy a cell must direct to DNA activities based on the arrangement of its genome and c) the costs and benefits of endosymbionts for cells of various volume.
It turns out that cells can grow to considerable volume and acquire at least some of the characteristics of complex cells while remaining primarily prokaryotic in character and without the presence of mitochondria.
The new picture of early eukaryote evolution provides a plausible alternative to the mitochondria-first paradigm. Rather than evolution ushering in the age of eukaryotes with one grand gesture — the chance acquisition of a mitochondrial prototype, a series of tentative, gradual, step-wise changes over vast timespans ultimately produced complex cells packed with sophisticated internal structures and capable of explosive diversification.
Earlier research by Lynch and Marinov cited in the new study takes a somewhat more radical view, implying that mitochondria offered few if any benefits to early eukaryotes. The new study stakes out a more moderate position, suggesting that beyond a critical cell volume, mitochondria and perhaps other features of modern eukaryotic cells would have been necessary to satisfy the energy needs of large cells, but a range of smaller proto-eukaryotes may have done just fine without these innovations.
Hence, the transition to the mysterious LECA event may have been preceded by a series of organisms, which may have initially been mitochondria-free.
The new research also throws into question the timing of eukaryotic transition events. Perhaps the great transition began with the development of a eukaryotic cytoskeleton or other advanced structure.
Much more research will be required to confidently place the series of events leading to fully-fledged eukaryotes in their proper sequence.Full article at Science Daily
From the 2nd paragraph: “The eukaryotes would evolve many complex internal modules or organelles, including the endoplasmic reticulum, the Golgi apparatus and the mitochondria, forming wildly diverse cell types — precursors to all subsequent plant and animal life on earth. Prokaryotic cells, which include bacteria and archaea, are structurally simple organisms, lacking the complex internal structure found in eukaryotes.” To acknowledge the vast increase in specific, functional biochemical complexity accompanying the origin of eukaryotes and to not even mention the constraints on information gain by any natural processes (step-wise or otherwise) is to simply be whistling Dixie in the dark.