快猫短视频

THE fossils scattered across the tabletop are an odd collection. One, called Cyclomedusa, resembles a fried egg. Spriggina looks like a grub wearing a helmet, Pteridinium like the tread of a bicycle tyre, and Dickinsonia has been described as a wad of gum squashed by a boot heel. These are the shadows of life forms that were once Earth鈥檚 most sophisticated inhabitants. They come from an idyllic age that ended abruptly about 545 million years ago.

鈥淭his,鈥� says Mark McMenamin, 鈥渋s the most dramatic moment in the history of life.鈥� The professor of geology from Mount Holyoke College in western Massachusetts, argues that it was at this moment in geological time that one thread of life was strangled by another. These strange beings, called Ediacarans (ee-dee-ACK-rins) were extinguished by our own forebears-animals. 鈥淓diacarans were on a trajectory in which they would have developed into intelligent life,鈥� claims McMenamin, 鈥渂ut it was cut short.鈥�

His controversial ideas heap new fuel on one of the most heated debates in palaeontology: just how do these oddballs fit into the evolutionary scheme of life on Earth? If McMenamin is correct, Ediacarans were neither animal nor plant but a unique kingdom of beings. 鈥淐omplex life evolved twice and only one form made it to the present day,鈥� says McMenamin.

But McMenamin鈥檚 is a maverick view. Many experts are not convinced that Ediacarans disappeared without a trace. They argue that there is a continuous line of evolution from Ediacarans to modern organisms, even though the links are missing from the fossil record. In other words, they believe that these strange creatures were prototype animals.

Named after the Ediacara Hills in South Australia, where the first major discovery of these fossils was made, Ediacarans popped up about 600 million years ago as knobs of protoplasm among the mats of mould and algae that were Earth鈥檚 main inhabitants at the time. These new creatures were the first sizeable multicellular Earthlings. They predated vascular plants by 200 million years and came to tower over the rudimentary worm-like animals that were their contemporaries.

From their humble origins, Ediacarans eventually colonised the ocean floors around most of the globe. None of their contemporaries were as large or elaborate in their physical structures. But despite their various forms, Ediacarans apparently had no teeth, claws, snouts or any of the physical features they might need to feed on their neighbours. Theirs seems to have been a world without predators, an uneventful paradise which McMenamin has called 鈥渢he Garden of Ediacara鈥�. The peaceful residents of this prehistoric Eden photosynthesised their own food, absorbed nutrients from passing waters or were home to the bacteria that fed them.

For tens of millions of years Ediacarans thrived in their watery idyll, developing into many shapes including fronds, bulbs and quilted mats that stretched to a metre long. Some would have been attached to the seabed, others were free-floating. A few Ediacarans such as Kimberella may even have had crude fins to propel themselves along the seabed. Then, about 550 million years ago, some began to sprout intriguing new body parts. Marywadea developed what looks like a simple antenna. Spriggina evolved outsize lobes at its largest end.

Crude brains

Some researchers have interpreted these as merely overdeveloped holdfasts. Many experts, however, argue that the growths are too elaborate for such a simple task. McMenamin goes even further. He boldly asserts that the novel appendages were concentrations of sensory tissue-perhaps sensitive to light or to chemical or temperature fluctuations in the water-that presage the development of awareness. The position and complexity of these structures only has one parallel that we know of-Ediacarans were growing crude brains. 鈥淭hey were developing ways to pick up environmental cues and process that information in ways that would allow them to adapt better and leave more progeny,鈥� he says. 鈥淓diacarans represent the first evidence of anything like intelligence on Earth.鈥�

Many researchers believe there just isn鈥檛 enough evidence to support this idea, but whatever the purpose of these strange appendages there is no doubt that just as Ediacarans were beginning to explore their potential, they disappeared abruptly from the fossil record. All trace of them vanished at the Cambrian explosion, a geological snap of the fingers when animal evolution went into hyperdrive. Some palaeontologists contend that geological upheavals clouded the transparent oceans, blotting out the Sun upon which Ediacarans depended for energy. McMenamin believes that new predators ate the Ediacarans out of existence. 鈥淣ewly evolved animals could sense their surroundings in some way, they could move and they could chew,鈥� he says. 鈥淭hey travelled around stuffing the world鈥檚 ecosystems into their mouths.鈥�

Other researchers argue that Ediacarans themselves were not lost, just the material that would have preserved them as fossils. The Precambrian seabeds were carpeted with mats of microbes that formed impressions of Ediacaran bodies just as a blob of putty yields to a fingerprint. During the Cambrian upheavals, those mats were rent by geological turmoil and by the new legions of worm-like animals burrowing into the seabeds. 鈥淚t鈥檚 as if you removed film from your camera,鈥� explains Guy Narbonne, professor of geology at Queens University in Kingston, Ontario. 鈥淭he object you want to photograph is still there, but you have no medium on which to record the image.鈥�

Despite the missing fossils, for decades the accepted view was that Ediacarans did survive and develop into some of the animals we see today. Forty years ago, Martin Glaessner from the University of South Australia at Adelaide, declared that most Ediacarans were animals-the crude versions of today鈥檚 jellyfish, sea pens and similarly simple beasts that occupy the fringes of life. Glaessner is generally seen as the father of Ediacaran science and his views remained unchallenged until 1982. Then Adolph Seilacher, a renowned German fossil scholar at the University of T眉bingen in Germany, announced that Ediacarans were not animals at all. Instead, he contended, they were in a now-extinct class by themselves-neither animal nor plant, but an extended family of creatures he called 鈥渧endobionts鈥� that inflated their bodies with water.

Seilacher鈥檚 claims triggered a free-for-all in palaeobiology, throwing interpretation of the Ediacaran fossil record wide open. Some researchers still see these strange creatures as prototype animals (see 鈥淢y cousin blob鈥�), others claim they were lichen-a mixture of fungi and algae-and a third view splits the group into ancestral animals and oddballs. McMenamin鈥檚 analysis is typically radical. Like Seilacher, he classifies all Ediacarans in a kingdom of their own, but unlike Seilacher he is convinced that all Ediacarans were multicellular organisms. And this is at the root of the anatomy argument that McMenamin uses to back up his ideas.

鈥淰irtually all true Ediacarans are variations on a theme,鈥� he says. Their bodies are made up of vanes or lobes of repeating segments, with complex forms comprising as many as 10 vanes attached to a central axis. Even those individuals with a rather different anatomy were usually divided into distinct facets, each made up of a repeating pattern of segments. McMenamin sees this as the key to Ediacaran classification. He says that this repetition of a few simple components points to a form of development called metacellularity.

Developing idea

Most living things develop from a single cell that divides into two, four, eight, sixteen and so on, until eventually the cells start to grow into specific organs and structures. In metacellular organisms such as some modern algae, however, the initial cell splits just a few times to form a handful of cells. Each so-called founding cell then develops independently into a single 鈥渃ell family鈥� corresponding to one distinct portion of the organism. McMenamin explains the shape of every known Ediacaran in this way. The blob-like Cyclomedusa-probably the original Ediacaran-is the overgrown result of a single cell family. The flat Dickinsonia grew from two, joined at the hip as they added segments. And Spriggina grew from three founding cells, one making the midline and the other two making the rows of segments along it.

McMenamin鈥檚 ideas on Ediacaran development seem fairly uncontentious. But he makes an intellectual step that few of his colleagues are willing to accept. He argues that because Ediacarans are metacellular none of them can be animals.

Only animals begin life as a single cell that divides again and again to form a hollow sphere of cells known as a blastula. It is from this that animal architecture emerges. 鈥淭o believe that any Ediacaran is an animal, you鈥檇 have to believe that they went through a blastular stage which resulted in a metacellular body,鈥� he says. 鈥淎nd then, at the Cambrian explosion, suddenly the blastula completely changed the kind of anatomy it created. That鈥檚 not credible.鈥�

Here, at least, McMenamin has one influential supporter, Lynn Margulis from the University of Massachusetts. 鈥淚n today鈥檚 animals, blastulas go on to form a mouth, anus, nerves, the testes and ovaries needed to make new blastulas,鈥� she says. 鈥淎 blastula stands for all these traits that are typical of animals, and I see no evidence for the fundamental aspects of blastula-ness in Ediacarans.鈥� She also points out that modern organisms classified as Protoctists-a separate kingdom of creatures that are neither animal nor plant-tend to have a similar anatomy to Ediacarans and none goes through a blastular stage in its development. 鈥淓ither Mark鈥檚 statement is correct,鈥� says Margulis, 鈥渙r there鈥檚 a lineage that goes from blastual to Ediacaran body form of which every trace is gone in all living and fossil organisms.鈥�

Fossil finds from China, published in February, provide the first concrete evidence that blastulas did exist at least 570 million years ago, a time when Ediacarans were still young. McMenamin has no problem accounting for these. 鈥淭here were animals-perhaps worms or sponges-leaving traces of themselves in the same rock formations as Ediacaran fossils,鈥� he says. 鈥淎ll of these forms would have been true animals with blastulas. That doesn鈥檛 mean that every complex creature alive in the earliest days of multicellular organisms had a blastula or was an animal.鈥� But researchers who believe that Ediacarans were animals interpret these findings differently. Richard Jenkins from the University of Adelaide applies Occam鈥檚 razor, arguing that if nature had already found an efficient way to grow multicellular organisms-the blastula-why would it evolve, in parallel, a more limited way to accomplish the same goal?

James Gehling, a geologist at the University of South Australia, also doubts McMenamin鈥檚 claims. 鈥淏efore you decide that something is a new class of organism, you first explore all other possibilities until they fail,鈥� he says. 鈥淪ome Ediacarans such as Pteridinium may indeed be in a class by themselves, but others are so close to Cambrian forms that one cannot avoid the conclusion that we鈥檙e dealing with ancestor-descendant relationships.鈥�

Jelly and wood

Such differences are revealed by the types of fossil impressions Ediacarans have left. These suggest that some species had delicate, soft bodies while others were much more robust. 鈥淲hat I see written very clearly in these fossils,鈥� says Narbonne, 鈥渋s that Dickinsonia and Cyclomedusa were made of things as different as jelly and wood. You can鈥檛 put those two materials in the same family in the modern world, so why would you assume you could in the ancient one?鈥� Like Gehling, he concludes that some Ediacarans did develop into animals, while others are distinctly different from living forms. 鈥淲e don鈥檛 yet have enough evidence to classify things like Spriggina or Marywadea,鈥� adds Narbonne.

So what does this mean for McMenamin鈥檚 theories? Suppose some Ediacarans did evolve into modern animals, that still leaves a group of organisms whose nature and fate remain unaccounted for. As Narbonne points out, this group may include Spriggina and Marywadea, two prime examples of organisms that seemed to be growing in complexity shortly before the Cambrian explosion. Even researchers who accept this, however, balk at McMenamin鈥檚 assertion that Ediacarans were geared up to develop into intelligent beings.

He draws energy for this argument from his belief that evolution is purposeful rather than being a chance affair. 鈥淲e see a very strong force-probably something as strong as natural selection itself and sparked by natural selection-inducing more complex forms of life that can draw and organise sensory information from the environment and use it to become increasingly more adaptable to the environment,鈥� he says. 鈥淟ife evokes mind.鈥�

This is a direct challenge to current thinking about the way evolution works. 鈥淰ery simple organisms don鈥檛 have much else to do but become more complex over time. But there are a number of instances in which complex animals have become simpler,鈥� says Stephen Rowland, a geologist from the University of Nevada at Las Vegas. 鈥淭here鈥檚 no necessary arrow of increased complexity through time.鈥� Indeed, if Ediacarans really were developing sensory organs this could be seen as evidence of their animal nature. 鈥淭he idea that two entirely different living systems happened to develop intelligence, based on entirely different cellular arrangements and processes, would seem to be incredibly unlikely in the light of what we鈥檙e learning about genetics,鈥� says Gehling.

McMenamin is unrepentant. 鈥淢y work changes everything in the debate,鈥� he says. 鈥淚t shows vitalism in which the forces and structures of the Universe evoke life, which evokes complex life, which evokes intelligence.鈥� The cosmic implication of this is that intelligent life must be much more common on other planets than people had previously suspected.

But even those who are attracted to this notion are wary of extrapolating from a rare event on one planet. 鈥淚f we had others where the same thing has happened, then we could say clearly that something about life drives towards complexity,鈥� says Rudolf Raff, a biologist from the University of Indiana. As it is, he concludes that the evolution of complex body forms is not inevitable but that when it happens it creates a positive feedback loop of some kind. 鈥淥nce a nervous system evolves, it becomes a driver for natural selection-to outcompete other guys with nervous systems or just to avoid being eaten by them.鈥� We shall never know whether Ediacarans would have evolved in this way, but Raff鈥檚 idea does provide an explanation for the meteoric rise of animals during the Cambrian explosion.