The Oldest Traditional Buddhist Lineage?

Like you said gurus exist only in hinduism and jainism. Gurus are complete non-sense in buddhism because the bikkhus move around a lot, so they cannot have followers (bikkhus or lay people). For instance, a lay person (buddhist or not) who gives food to a bikkhu may never see this bhikku ever again after a few days of offering. However a bikkhu may have special link with the bikkhu who ordained him. It is expected that ''The preceptor will foster the attitude he would have toward a son (‘son-mind’) with regard to the student. The student will foster the attitude he would have toward a father (‘father-mind’) with regard to the preceptor. Thus they—living with mutual respect, deference, and courtesy—will arrive at growth, increase, and maturity in this Dhamma-Vinaya.'' https://www.dhammatalks.org/vinaya/bmc/Section0054.html Bikkhu really are nomads., but a bikkhu may live in the forest or go from monasteries to monasteries, there is no rule binding a bikkhu to one monastery.

The other idea from the hindus is the one of devotion of the follower towards the guru, but being devoted is non-sensical again in buddhism, not at all part of the path (the path is just watching the mind to keep having only good intentions and good actions) and that the guru provides custom teaching fitting perfectly the follower, but again that's not what happens in buddhism.

For instance the monk Channa https://suttacentral.net/sn22.90/en/sujato had trouble to reach full awakening, so he went from ''senior mendicants '' to ''senior mendicants '' and they said the generic teaching that the buddha keep saying, he still did not get it though. So he went to Ananda who told him again a generic teaching and he said that he ''has comprehended the teaching.'' So the monk Channa did not stick to the first ''senior mendicants '' he met and made them his guru for many years and being devoted to them like they do in hinduism. Since he did not understand anything from them, he just went to other ''senior mendicants '' until he understood whatever he wanted. This is how it is supposed to work in buddhism when there are arahants around: going to live with a few arahants until you get it, mostly because the arahants from which you understand buddhism click with you.

Currently there is only claims about having a direct lineage to the buddha. The best you can have is the theravadans who say their ancestors started writing the suttas in 100BC, mostly in the south of india (moslty lanka), and before that the teaching was only oral and indeed directly from the buddha.

Since there is no gurus in theravada, the theravadas do not care about lineage and gurus. As long as the bikkhus base their study on the suttas and optionnaly on the theravadan abiddhama, they are happy..

So the answer is that you wont find a way to determine if somebody is indeed in a sect directly coming from the time of the buddha. Perhaps even the theravada have modified too much the teachings. Perhaps the buddha did not even exist, because today historians say the buddha really existed because ''we cannot explain how so many suttas popped up all over india with lots of almost exact copies, so the buddha must have really lived''.

Lineage exists both in zen Buddhism and vajrayana practices, [I've read these called perfection and tantric teachings]. In the latter, key names include vajrabodhi, amoghavajra, and subhakarasimha; esoteric Japanese Buddhists were able to visit China and succeed earlier patriarchs in much the same way as Zen Buddhists.

e.g., Saicho, who brought Tendai to Japan and was a friend and rival of kukai, who brought shingon to Japan, received "transmission" in the northern school from gyohyo, the ox head "transmission" from hsiu-jan, and esoteric

transmission from Weixiang of Guochingsi on Mount Tiantai

And scholastic Buddhists like hua-yen [beginning with Dushun] and tientai [beginning with Nagarjuna] Buddhists also have lineages and patriarchs, though there is not always the necessity to meet and be initiated as there is in zen and esoteric Buddhism.

Huiwen (Chinese: 慧文) is traditionally considered to be the second patriarch of the Tiantai school. Huiwen studied the works of Nāgārjuna, and is said to have awakened to the profound meaning of Nāgārjuna's words: "All conditioned phenomena I speak of as empty, and are but false names which also indicate the mean."

So, it depends on what you mean by "guru". There are important Buddhist teachers, both historically and today, and sometimes one needs to meet them to be initiated into their tradition.

I found this interesting passage from this Wikipedia page:

There are several rules in the Theravada monastic code by which a bhikkhu is "defeated" - he is no longer a bhikkhu even if he continues to wear robes and is treated as one. Every ordination ceremony in Theravada Buddhism is performed by ten bhikkhus to guard against the possibility of the ordination being rendered invalid by having a "defeated bhikkhu" as preceptor.

I've personally never heard of this, but apparently ordination of new monks are done by multiple monks, to ensure that at least one of the monks performing the ordination is not "defeated". This is to ensure that every ordination can be traced back all the way to the Buddha. That said, I doubt that a detailed record of the lineage is kept.

I would say that for Theravada Buddhism, the lineage of monastic ordination is important (without keeping meticulous records of lineage), but not the lineage of teaching.

As the Buddha has stated in the Mahaparinibbana Sutta (quoted below), he is not leaving behind any successor. Instead, his followers must depend on themselves, with the Dhamma (the Buddha's teachings) as their basis. There is no mandatory need for a teacher as an intermediary (although the Buddha did specify criteria for a Dhamma teacher in the Udayi Sutta). The Buddha also did not hide any esoteric teachings. Due to this, a lineage of teaching is not important.

Thus spoke the Venerable Ananda, but the Blessed One answered him, saying: "What more does the community of bhikkhus expect from me, Ananda? I have set forth the Dhamma without making any distinction of esoteric and exoteric doctrine; there is nothing, Ananda, with regard to the teachings that the Tathagata holds to the last with the closed fist of a teacher who keeps some things back. Whosoever may think that it is he who should lead the community of bhikkhus, or that the community depends upon him, it is such a one that would have to give last instructions respecting them. But, Ananda, the Tathagata has no such idea as that it is he who should lead the community of bhikkhus, or that the community depends upon him. So what instructions should he have to give respecting the community of bhikkhus?

"Now I am frail, Ananda, old, aged, far gone in years. This is my eightieth year, and my life is spent. Even as an old cart, Ananda, is held together with much difficulty, so the body of the Tathagata is kept going only with supports. It is, Ananda, only when the Tathagata, disregarding external objects, with the cessation of certain feelings, attains to and abides in the signless concentration of mind, that his body is more comfortable.

"Therefore, Ananda, be islands unto yourselves, refuges unto yourselves, seeking no external refuge; with the Dhamma as your island, the Dhamma as your refuge, seeking no other refuge.

"And how, Ananda, is a bhikkhu an island unto himself, a refuge unto himself, seeking no external refuge; with the Dhamma as his island, the Dhamma as his refuge, seeking no other refuge?

"When he dwells contemplating the body in the body, earnestly, clearly comprehending, and mindfully, after having overcome desire and sorrow in regard to the world; when he dwells contemplating feelings in feelings, the mind in the mind, and mental objects in mental objects, earnestly, clearly comprehending, and mindfully, after having overcome desire and sorrow in regard to the world, then, truly, he is an island unto himself, a refuge unto himself, seeking no external refuge; having the Dhamma as his island, the Dhamma as his refuge, seeking no other refuge.

"Those bhikkhus of mine, Ananda, who now or after I am gone, abide as an island unto themselves, as a refuge unto themselves, seeking no other refuge; having the Dhamma as their island and refuge, seeking no other refuge: it is they who will become the highest, if they have the desire to learn."

I will tell what i know of Theravada.

Afaik Theravadin Abhidhamma order was established on the Ceylon, Sri Lanka south of India 300 BC, this was still in the period Buddha proclaimed to be of True Dhamma and the culture was thriving for about 1500 years. No other of the early lineages from the true Dhamma period and no Vibhajavada school in particular remained thriving for as long as the Theravada on SL.

This is in part due to the unique geo location.

It started first to decline after an invasion in the 12th century by Kalinga Magha, the Theravadin order was eventually more or less extinct in the 17th century.

The Sri Lankan order was revived with ordination by Burmese Theravadin monks. These were of the exact same Theravada lineage which was becoming extinct in SL as the SL missionaries spread it to Burma region to begin with.

Afaik the early missionaries maybe could've reached that Burmese region early some 200 BC but idk.

There are artifacts from ~ the 1st century and we know abhidhamma was studied in that region around 5-7th century.

Theravadin tradition became widespread around the 10th century and they had contact with Sri Lankans but the Sangha was by then quite fragmented and with non canonical 7th century commentaries. The Sangha was eventually forcibly hard reformed in the late 14th century and eventually the Sangha again became more Abhidhamma focused and they help revived the Sri Lankan order.

https://www.accesstoinsight.org/lib/authors/bischoff/wheel399.html#ch1 on history in Burmese region

The Six qualities of the Dhamma:

1. Svakkhato: The Dhamma is not a speculative philosophy, but is the Universal Law found through enlightenment and is preached precisely. Therefore it is Excellent in the beginning (Sila — Moral principles), Excellent in the middle (Samadhi — Concentration) and Excellent in the end (Panna — Wisdom),

2. Samditthiko: The Dhamma is testable by practice and known by direct experience,

3. Akaliko: The Dhamma is able to bestow timeless and immediate results here and now, for which there is no need to wait until the future or next existence.

4. Ehipassiko: The Dhamma welcomes all beings to put it to the test and to experience it for themselves.

5. Opaneyiko: The Dhamma is capable of being entered upon and therefore it is worthy to be followed as a part of one's life.

6. Paccattam veditabbo vinnunhi: The Dhamma may be perfectly realized only by the noble disciples who have matured and enlightened enough in supreme wisdom.

(from Anguttara Nikaya 11.12)

Source: 6 qualities of the Dhamma

Also, the progression of the Dhamma is lean the theory - Pariyatti (theory), put it into practice - Patipatti (practice), and finally realise the results - Pativedha (realisation).

The qualities of Dhamma are to be experienced and realised 1st hand culminating in the direct confirmation of the theory. Hence the quality of the results and experience in verifying the teachings can imply what tradition is closest to that of the Buddha and handed down in the purest form. The proof of the pudding is in the eating.